What we need is a widget which presents a collection of buttons.  The user
can select only one of them at a time.  When one is selected, its
foreground and background colors should be reversed and stay that way
until another one is selected.  Exactly one button can be selected at
any one time.

Possible implementations:

1.  As a new kind of widget.  bleah.


2.  Box surrounding a lot of command buttons.  The command buttons have
    the appropriate callbacks.

Callbacks:  a) set all buttons to normal colors
            b) set this button to reverse colors
            c) change the appropriate variable to the desired value.

    This can all be done at the time of creation, thus the buttons would
    be invisible to the rest of the program (a good thing).



(2) would be a heck of a lot easier to program, since creating widgets is
a bitch.  However, (1) could come in handy outside of this program.  Perhaps
write it using (2) and switch to (1) if/when we can get the widget working.