doc: update documentation

No more GrlMediaFoos.

https://bugzilla.gnome.org/show_bug.cgi?id=755551

9 files changed, 120 insertions, 217 deletions

diff --git a/doc/grilo/grilo-docs.sgml b/doc/grilo/grilo-docs.sgml
index 7054e69..4879123 100644
--- a/doc/grilo/grilo-docs.sgml
+++ b/doc/grilo/grilo-docs.sgml
@@ -95,10 +95,6 @@
       <xi:include href="xml/grl-data.xml"/>
       <xi:include href="xml/grl-related-keys.xml"/>
       <xi:include href="xml/grl-media.xml"/>
-      <xi:include href="xml/grl-media-box.xml"/>
-      <xi:include href="xml/grl-media-video.xml"/>
-      <xi:include href="xml/grl-media-audio.xml"/>
-      <xi:include href="xml/grl-media-image.xml"/>
     </chapter>
 
     <chapter id="misc">
diff --git a/doc/grilo/grilo-sections.txt b/doc/grilo/grilo-sections.txt
index c767a41..e15189c 100644
--- a/doc/grilo/grilo-sections.txt
+++ b/doc/grilo/grilo-sections.txt
@@ -340,31 +340,74 @@ GrlMediaClass
 GrlMediaSerializeType
 GrlMediaType
 grl_media_new
+grl_media_audio_new
+grl_media_image_new
+grl_media_video_new
+grl_media_container_new
+grl_media_is_audio
+grl_media_is_image
+grl_media_is_video
+grl_media_is_container
+grl_media_add_artist
 grl_media_add_author
+grl_media_add_director
 grl_media_add_external_player
 grl_media_add_external_url
+grl_media_add_genre
 grl_media_add_keyword
+grl_media_add_lyrics
+grl_media_add_mb_artist_id
+grl_media_add_performer
+grl_media_add_producer
 grl_media_add_region_data
 grl_media_add_thumbnail
 grl_media_add_thumbnail_binary
 grl_media_add_url_data
+grl_media_get_album
+grl_media_get_artist
+grl_media_get_artist_nth
 grl_media_get_author
 grl_media_get_author_nth
+grl_media_get_bitrate
+grl_media_get_camera_model
 grl_media_get_certificate
+grl_media_get_childcount
 grl_media_get_creation_date
 grl_media_get_description
+grl_media_get_director
+grl_media_get_director_nth
 grl_media_get_duration
+grl_media_get_episode
+grl_media_get_episode_title
+grl_media_get_exposure_time
 grl_media_get_favourite
+grl_media_get_flash_used
+grl_media_get_framerate
+grl_media_get_genre
+grl_media_get_genre_nth
+grl_media_get_height
 grl_media_get_external_url
 grl_media_get_external_url_nth
 grl_media_get_id
+grl_media_get_iso_speed
 grl_media_get_keyword
 grl_media_get_keyword_nth
 grl_media_get_last_played
 grl_media_get_last_position
 grl_media_get_license
+grl_media_get_mb_album_id
+grl_media_get_mb_artist_id
+grl_media_get_mb_artist_id_nth
+grl_media_get_mb_recording_id
+grl_media_get_mb_track_id
 grl_media_get_mime
 grl_media_get_modification_date
+grl_media_get_orientation
+grl_media_get_original_title
+grl_media_get_performer
+grl_media_get_performer_nth
+grl_media_get_producer
+grl_media_get_producer_nth
 grl_media_get_play_count
 grl_media_get_player
 grl_media_get_player_nth
@@ -373,6 +416,8 @@ grl_media_get_rating
 grl_media_get_region
 grl_media_get_region_data
 grl_media_get_region_data_nth
+grl_media_get_season
+grl_media_get_show
 grl_media_get_site
 grl_media_get_source
 grl_media_get_start_time
@@ -381,42 +426,71 @@ grl_media_get_thumbnail
 grl_media_get_thumbnail_binary
 grl_media_get_thumbnail_binary_nth
 grl_media_get_thumbnail_nth
+grl_media_get_track_number
 grl_media_get_title
 grl_media_get_url
 grl_media_get_url_data
 grl_media_get_url_data_nth
 grl_media_get_size
+grl_media_get_width
 grl_media_serialize
 grl_media_serialize_extended
+grl_media_set_album
+grl_media_set_artist
 grl_media_set_author
+grl_media_set_bitrate
+grl_media_set_camera_model
 grl_media_set_certificate
+grl_media_set_childcount
 grl_media_set_creation_date
 grl_media_set_description
+grl_media_set_director
 grl_media_set_duration
+grl_media_set_episode
+grl_media_set_episode_title
+grl_media_set_exposure_time
 grl_media_set_external_player
 grl_media_set_external_url
 grl_media_set_favourite
+grl_media_set_flash_used
+grl_media_set_framerate
+grl_media_set_genre
+grl_media_set_height
 grl_media_set_id
+grl_media_set_iso_speed
 grl_media_set_keyword
 grl_media_set_last_played
 grl_media_set_last_position
 grl_media_set_license
+grl_media_set_lyrics
+grl_media_set_mb_album_id
+grl_media_set_mb_artist_id
+grl_media_set_mb_recording_id
+grl_media_set_mb_track_id
 grl_media_set_mime
 grl_media_set_modification_date
+grl_media_set_original_title
+grl_media_set_orientation
+grl_media_set_performer
+grl_media_set_producer
 grl_media_set_play_count
 grl_media_set_publication_date
 grl_media_set_rating
 grl_media_set_region
 grl_media_set_region_data
+grl_media_set_season
+grl_media_set_show
 grl_media_set_site
 grl_media_set_source
 grl_media_set_studio
 grl_media_set_thumbnail
 grl_media_set_thumbnail_binary
 grl_media_set_title
+grl_media_set_track_number
 grl_media_set_url
 grl_media_set_url_data
 grl_media_set_size
+grl_media_set_width
 grl_media_unserialize
 <SUBSECTION Standard>
 GRL_IS_MEDIA
@@ -477,156 +551,6 @@ GrlDataPrivate
 </SECTION>
 
 <SECTION>
-<FILE>grl-media-box</FILE>
-<TITLE>GrlMediaBox</TITLE>
-GrlMediaBox
-GrlMediaBoxClass
-grl_media_box_new
-grl_media_box_get_childcount
-grl_media_box_set_childcount
-<SUBSECTION Standard>
-GRL_IS_MEDIA_BOX
-GRL_IS_MEDIA_BOX_CLASS
-GRL_MEDIA_BOX
-GRL_MEDIA_BOX_CLASS
-GRL_MEDIA_BOX_GET_CLASS
-GRL_TYPE_MEDIA_BOX
-grl_media_box_get_type
-</SECTION>
-
-<SECTION>
-<FILE>grl-media-video</FILE>
-<TITLE>GrlMediaVideo</TITLE>
-GrlMediaVideo
-GrlMediaVideoClass
-grl_media_video_new
-grl_media_video_add_director
-grl_media_video_add_performer
-grl_media_video_add_producer
-grl_media_video_add_url_data
-grl_media_video_get_director
-grl_media_video_get_director_nth
-grl_media_video_get_episode
-grl_media_video_get_episode_title
-grl_media_video_get_framerate
-grl_media_video_get_height
-grl_media_video_get_original_title
-grl_media_video_get_performer
-grl_media_video_get_performer_nth
-grl_media_video_get_producer
-grl_media_video_get_producer_nth
-grl_media_video_get_season
-grl_media_video_get_show
-grl_media_video_get_url_data
-grl_media_video_get_url_data_nth
-grl_media_video_get_width
-grl_media_video_set_director
-grl_media_video_set_episode
-grl_media_video_set_episode_title
-grl_media_video_set_framerate
-grl_media_video_set_height
-grl_media_video_set_original_title
-grl_media_video_set_performer
-grl_media_video_set_producer
-grl_media_video_set_season
-grl_media_video_set_show
-grl_media_video_set_size
-grl_media_video_set_url_data
-grl_media_video_set_width
-<SUBSECTION Standard>
-GRL_IS_MEDIA_VIDEO
-GRL_IS_MEDIA_VIDEO_CLASS
-GRL_MEDIA_VIDEO
-GRL_MEDIA_VIDEO_CLASS
-GRL_MEDIA_VIDEO_GET_CLASS
-GRL_TYPE_MEDIA_VIDEO
-grl_media_video_get_type
-</SECTION>
-
-<SECTION>
-<FILE>grl-media-audio</FILE>
-<TITLE>GrlMediaAudio</TITLE>
-GrlMediaAudio
-GrlMediaAudioClass
-grl_media_audio_new
-grl_media_audio_add_artist
-grl_media_audio_add_genre
-grl_media_audio_add_lyrics
-grl_media_audio_add_mb_artist_id
-grl_media_audio_add_url_data
-grl_media_audio_get_album
-grl_media_audio_get_artist
-grl_media_audio_get_artist_nth
-grl_media_audio_get_bitrate
-grl_media_audio_get_genre
-grl_media_audio_get_genre_nth
-grl_media_audio_get_lyrics
-grl_media_audio_get_lyrics_nth
-grl_media_audio_get_mb_album_id
-grl_media_audio_get_mb_artist_id
-grl_media_audio_get_mb_artist_id_nth
-grl_media_audio_get_mb_recording_id
-grl_media_audio_get_mb_track_id
-grl_media_audio_get_track_number
-grl_media_audio_get_url_data
-grl_media_audio_get_url_data_nth
-grl_media_audio_set_album
-grl_media_audio_set_artist
-grl_media_audio_set_bitrate
-grl_media_audio_set_genre
-grl_media_audio_set_lyrics
-grl_media_audio_set_mb_album_id
-grl_media_audio_set_mb_artist_id
-grl_media_audio_set_mb_recording_id
-grl_media_audio_set_mb_track_id
-grl_media_audio_set_track_number
-grl_media_audio_set_url_data
-<SUBSECTION Standard>
-GRL_IS_MEDIA_AUDIO
-GRL_IS_MEDIA_AUDIO_CLASS
-GRL_MEDIA_AUDIO
-GRL_MEDIA_AUDIO_CLASS
-GRL_MEDIA_AUDIO_GET_CLASS
-GRL_TYPE_MEDIA_AUDIO
-grl_media_audio_get_type
-</SECTION>
-
-<SECTION>
-<FILE>grl-media-image</FILE>
-<TITLE>GrlMediaImage</TITLE>
-GrlMediaImage
-GrlMediaImageClass
-grl_media_image_new
-grl_media_image_add_url_data
-grl_media_image_get_camera_model
-grl_media_image_get_exposure_time
-grl_media_image_get_flash_used
-grl_media_image_get_height
-grl_media_image_get_iso_speed
-grl_media_image_get_orientation
-grl_media_image_get_url_data
-grl_media_image_get_url_data_nth
-grl_media_image_get_width
-grl_media_image_set_camera_model
-grl_media_image_set_exposure_time
-grl_media_image_set_flash_used
-grl_media_image_set_height
-grl_media_image_set_iso_speed
-grl_media_image_set_orientation
-grl_media_image_set_size
-grl_media_image_set_url_data
-grl_media_image_set_width
-<SUBSECTION Standard>
-GRL_IS_MEDIA_IMAGE
-GRL_IS_MEDIA_IMAGE_CLASS
-GRL_MEDIA_IMAGE
-GRL_MEDIA_IMAGE_CLASS
-GRL_MEDIA_IMAGE_GET_CLASS
-GRL_TYPE_MEDIA_IMAGE
-grl_media_image_get_type
-</SECTION>
-
-<SECTION>
 <FILE>grl-metadata-key</FILE>
 GrlKeyID
 GRLKEYID_TO_POINTER
diff --git a/doc/grilo/grilo.types b/doc/grilo/grilo.types
index 24b9ba2..ce9358c 100644
--- a/doc/grilo/grilo.types
+++ b/doc/grilo/grilo.types
@@ -12,10 +12,6 @@ grl_data_get_type
 grl_config_get_type
 grl_related_keys_get_type
 grl_media_get_type
-grl_media_box_get_type
-grl_media_audio_get_type
-grl_media_video_get_type
-grl_media_image_get_type
 grl_plugin_get_type
 grl_source_get_type
 grl_registry_get_type
diff --git a/doc/grilo/plugins-sources.xml b/doc/grilo/plugins-sources.xml
index 81f09a9..3939ab5 100644
--- a/doc/grilo/plugins-sources.xml
+++ b/doc/grilo/plugins-sources.xml
@@ -455,16 +455,16 @@ grl_foo_source_search (GrlSource *source, GrlSourceSearchSpec *ss)
 
     <para>
       Browsing is an interactive process, where users navigate by exploring
-      these boxes exposed by the media source in hierarchical form. The idea of
-      browsing a media source is the same as browsing a file system.
+      these containers exposed by the media source in hierarchical form. The
+      idea of browsing a media source is the same as browsing a file system.
     </para>
 
     <para>
       The signature and way of operation of the Browse operation is the same as
       in the Search operation with one difference: instead of a text parameter
       with the search keywords, it receives a <link
-      linkend="GrlMedia">GrlMedia</link> object representing the container (box)
-      the user wants to browse.
+      linkend="GrlMedia">GrlMedia</link> object representing the container the
+      user wants to browse.
     </para>
 
     <para>
@@ -505,11 +505,11 @@ foo_execute_categories_async_cb (gchar *xml, GrlSourceBrowseSpec *bs)
   } else {
     /* parse_next parses the next category item in the XML
        and creates a GrlMedia instance with the data extracted,
-       which should be of type GrlMediaBox */
+       which should be a container */
     while (media = parse_next_cat (xml))
       bs->callback (bs->source,       /* Source emitting the data */
                     bs->operation_id, /* Operation identifier */
-                    media,            /* The category (box)  */
+                    media,            /* The category container  */
                     --count,          /* Remaining count */
                     bs->user_data,    /* User data for the callback */
                     NULL);            /* GError instance (if an error occurred) */
@@ -530,9 +530,7 @@ foo_execute_media_async_cb (gchar *xml, GrlSourceBrowseSpec *os)
                   NULL, 0, bs->user_data, NULL);
   } else {
     /* parse_next parses the next media item in the XML
-       and creates a GrlMedia instance with the data extracted,
-       which should be of type GrlMediaImage, GrlMediaAudio or
-       GrlMediaVideo */
+       and creates a GrlMedia instance with the data extracted */
     while (media = parse_next_media (xml))
       os->callback (os->source,       /* Source emitting the data */
                     os->operation_id, /* Operation identifier */
@@ -549,10 +547,10 @@ grl_foo_source_browse (GrlSource *source, GrlSourceBrowseSpec *bs)
   gchar *foo_http_browse:
 
   /* We use the names of the categories as their media identifiers */
-  box_id = grl_media_get_id (bs->container);
+  container_id = grl_media_get_id (bs->container);
 
-  if (!box_id) {
-    /* Browsing the root box, the result must be the list of
+  if (!container_id) {
+    /* Browsing the root container, the result must be the list of
        categories provided by the service */
     foo_http_browse =
       g_strdup_printf("http://media.foo.com/category_list",
@@ -565,7 +563,7 @@ grl_foo_source_browse (GrlSource *source, GrlSourceBrowseSpec *bs)
     /* Browsing a specific category */
     foo_http_browse =
       g_strdup_printf("http://media.foo.com/content/%s?offset=%d&count=%d",
-                      box_id,
+                      container_id,
                       grl_operation_options_get_skip (bs),
                       grl_operation_options_get_count (bs));
      /* This executes an async http query and then invokes
@@ -582,22 +580,20 @@ grl_foo_source_browse (GrlSource *source, GrlSourceBrowseSpec *bs)
         <listitem>
           In the example we are assuming that the content hierarchy only has two
           levels, the first level exposes a list of categories (each one exposed
-          as a <link linkend="GrlMediaBox">GrlMediaBox</link> object so the user
-          knows they can be browsed again), and then a second level with the
-          contents within these categories, that we assume are all media items,
-          although in real life they could very well be more <link
-          linkend="GrlMediaBox">GrlMediaBox</link> objects, leading to more
-          complex hierarchies.
+          as a container <link linkend="GrlMedia">GrlMedia</link> object so the
+          user knows they can be browsed again), and then a second level with
+          the contents within these categories, that we assume are all media
+          items, although in real life they could very well be more containers,
+          leading to more complex hierarchies.
         </listitem>
         <listitem>
-          <link linkend="GrlMediaBox">GrlMediaBox</link> objects returned by a
-          browse operation can be browsed by clients in future Browse
-          operations.
+          Containers returned by a browse operation can be browsed by clients in
+          future Browse operations.
         </listitem>
         <listitem>
-          The input parameter that informs the plugin about the box that should
+          The input parameter that informs the plugin about the container that should
           be browsed (bs->container) is of type <link
-          linkend="GrlMediaBox">GrlMediaBox</link>. The plugin developer must
+          linkend="GrlMedia">GrlMedia</link>. The plugin developer must
           map that to something the media provider understands. Typically, when
           <link linkend="GrlMedia">GrlMedia</link> objects are returned from a
           plugin to the client, they are created so their "id" property (<link
@@ -606,8 +602,8 @@ grl_foo_source_browse (GrlSource *source, GrlSourceBrowseSpec *bs)
           context of the media provider.
         </listitem>
         <listitem>
-          A <link linkend="GrlMediaBox">GrlMediaBox</link> object with NULL id
-          always represents the root box/category in the content hierarchy
+          A <link linkend="GrlMedia">GrlMedia</link> object with NULL id always
+          represents the root container/category in the content hierarchy
           exposed by the plugin.
         </listitem>
       </itemizedlist>
@@ -746,13 +742,8 @@ grl_foo_source_query (GrlSource *source, GrlSourceQuerySpec *qs)
           to do in chunks using the idle loop is encouraged.
         </listitem>
         <listitem>
-          Creating <link linkend="GrlMedia">GrlMedia</link> instances is
-          easy, depending on the type of media you should instantiate one of the
-          <link linkend="GrlMedia">GrlMedia</link> subclasses (<link
-          linkend="GrlMediaImage">GrlMediaImage</link>, <link
-          linkend="GrlMediaVideo">GrlMediaVideo</link>, <link
-          linkend="GrlMediaAudio">GrlMediaAudio</link> or <link
-          linkend="GrlMediaBox">GrlMediaBox</link>), and then use the API to set the
+          Creating <link linkend="GrlMedia">GrlMedia</link> instances is easy,
+          you should instantiate one, and then use the API to set the
           corresponding data. Check the <link linkend="GrlData">GrlData</link>
           hierarchy in the API reference for more details.
         </listitem>
@@ -876,11 +867,11 @@ grl_foo_source_may_resolve (GrlSource *source,
 
   /* We only support audio items */
   if (media) {
-    if (!GRL_IS_MEDIA_AUDIO (media))
+    if (!grl_media_is_audio (media))
       return FALSE;
 
     /* We need artist information available */
-    if (grl_media_audio_get_artist (GRL_MEDIA_AUDIO (media)) == NULL) {
+    if (grl_media_get_artist (media) == NULL) {
       if (missing_keys)
         *missing_keys = g_list_add (*missing_keys,
                                     GRLKEYID_TO_POINTER (GRL_METADATA_KEY_ARTIST));
@@ -888,7 +879,7 @@ grl_foo_source_may_resolve (GrlSource *source,
     }
 
     /* We need album information available */
-    if (grl_media_audio_get_album (GRL_MEDIA_AUDIO (media)) == NULL)) {
+    if (grl_media_get_album (media) == NULL)) {
       if (missing_keys)
         *missing_keys = g_list_add (*missing_keys,
                                     GRLKEYID_TO_POINTER (GRL_METADATA_KEY_ALBUM));
@@ -912,8 +903,8 @@ grl_foo_source_resolve (GrlSource *source,
   const GError *error = NULL;
 
   if (contains_key (rs->keys, GRL_METADATA_KEY_THUMBNAIL) {
-    artist = grl_media_audio_get_artist (GRL_MEDIA_AUDIO (rs->media));
-    album = grl_media_audio_get_album (GRL_MEDIA_AUDIO (rs->media));
+    artist = grl_media_get_artist (rs->media);
+    album = grl_media_get_album (rs->media);
     if (artist && album) {
       thumb_uri = resolve_thumb_uri (artist, album);
       grl_media_set_thumbnail (rs->media, thumb_uri);
diff --git a/doc/grilo/writing-apps.xml b/doc/grilo/writing-apps.xml
index 0e96ac8..3f8a86b 100644
--- a/doc/grilo/writing-apps.xml
+++ b/doc/grilo/writing-apps.xml
@@ -303,12 +303,11 @@
       </para>
       
       <para>
-        Grilo also supports the concept of media container,
-        represented by the  GrlMediaBox subclass. These objects
-        represent categories or folders that contain other
-        media objects (including maybe more GrlMediaBox objects),
-        allowing tree-like hierarchies of content, like the ones
-        would usually traverse during Browse operations.
+        Grilo also supports the concept of media container, represented by a
+        container-type GrlMedia class. These objects represent categories or
+        folders that contain other media objects (including maybe more container
+        objects), allowing tree-like hierarchies of content, like the ones would
+        usually traverse during Browse operations.
       </para>
 
       <para>
@@ -516,11 +515,11 @@
         Grilo provides application and plugin developers with a high-level APIs to
         handle certain relations among keys consistently. Continuing with the example
         of the video resource with multiple URIs, there is
-        <link linkend="grl-media-video-add-url-data">
-          grl_media_video_add_url_data
+        <link linkend="grl-media-add-url-data">
+          grl_media_add_url_data
         </link> and
-        <link linkend="grl-media-video-add-url-data-nth">
-          grl_media_video_get_url_data_nth
+        <link linkend="grl-media-add-url-data-nth">
+          grl_media_get_url_data_nth
         </link> to add and retrieve all the metadata
         associated with a particular instance of the video resource (URI, mime-type,
         framerate, width and height, etc) in one go.
@@ -535,7 +534,7 @@
       <para>
         Here is a small program illustrating how get all available URLs from
         a video resource, as well the corresponding MIME value for each one.
-        We use GrlRelatedKeys instead of the high-level API from GrlMediaVideo
+        We use GrlRelatedKeys instead of the high-level API from GrlMedia
         to illustrate how to use it:
       </para>
       <programlisting role="C">
diff --git a/src/data/grl-data.c b/src/data/grl-data.c
index b453694..db8bdfb 100644
--- a/src/data/grl-data.c
+++ b/src/data/grl-data.c
@@ -26,13 +26,11 @@
  * SECTION:grl-data
  * @short_description: Low-level class for storing metadata information about
  * media resources provided by #GrlSource objects.
- * @see_also: #GrlMedia, #GrlMediaBox, #GrlMediaVideo, #GrlMediaAudio,
- * #GrlMediaImage
+ * @see_also: #GrlMedia
  *
- * This class acts as a generic metadata dictionary where metadata keys
- * and their values can be stored.
- * Usually, application and plugin developers would interact with specific
- * subclasses of #GrlData, like #GrlMedia or #GrlMediaVideo, which provide
+ * This class acts as a generic metadata dictionary where metadata keys and
+ * their values can be stored.  Usually, application and plugin developers would
+ * interact with specific subclass of #GrlData, #GrlMedia, which provide
  * specific API to manipulate well known attributes of these media types.
  */
 
diff --git a/src/data/grl-media.c b/src/data/grl-media.c
index 456db54..deaed68 100644
--- a/src/data/grl-media.c
+++ b/src/data/grl-media.c
@@ -25,8 +25,7 @@
 /**
  * SECTION:grl-media
  * @short_description: A multimedia data transfer object
- * @see_also: #GrlData, #GrlMediaBox, #GrlMediaVideo, #GrlMediaAudio,
- * #GrlMediaImage
+ * @see_also: #GrlData
  *
  * This high level class represents a multimedia item. It has methods to
  * set and get properties like author, title, description, and so on.
diff --git a/src/grl-caps.h b/src/grl-caps.h
index e3298f7..8dbdbdc 100644
--- a/src/grl-caps.h
+++ b/src/grl-caps.h
@@ -69,7 +69,7 @@ typedef struct {
 
 /**
  * GrlTypeFilter:
- * @GRL_TYPE_FILTER_NONE: allow no content (only #GrlMediaBox)
+ * @GRL_TYPE_FILTER_NONE: allow no content (only containers)
  * @GRL_TYPE_FILTER_AUDIO: allow audio content
  * @GRL_TYPE_FILTER_VIDEO: allow video content
  * @GRL_TYPE_FILTER_IMAGE: allow image content
diff --git a/src/grl-source.h b/src/grl-source.h
index f8e9ccb..68c1342 100644
--- a/src/grl-source.h
+++ b/src/grl-source.h
@@ -129,7 +129,7 @@ typedef enum {
 /**
  * GrlSourceChangeType:
  * @GRL_CONTENT_CHANGED: content has changed. It is used when any property of
- * #GrlMedia has changed, or in case of #GrlMediaBox, if several children have
+ * #GrlMedia has changed, or in case of containers, if several children have
  * been added and removed.
  * @GRL_CONTENT_ADDED: new content has been added.
  * @GRL_CONTENT_REMOVED: content has been removed