tubes: add a shared TeleManager singleton

Yes, this is in addition to the existing TeleManagerImpl singleton. This
class needs to be properly split in half: one Manager part from which
the UI can request new sessions and which signals the appearance of new
incoming sessions, and another Session part representing the shared
editing session (which in turn owns one or more Conferences, which owns
exactly one tube, as now).  The Manager will dispatch incoming files to
the appropriate Conference by UUID or similar.

But for now, Michael is opening a new window with the received file,
so we want incoming and outgoing events to go to both windows so that it
works well enough for a demo.

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