Bring back the actions tab; we don't receive
changes anymore, since GtkActionMuxer lost
the GActionGroup signals for this, and the
action observer machinery has no way to listen
for all changes.
Instead of iterating action groups manually,
just show what the action muxer provides. This
gives an accurate view of what actions are
available in a widgets context, and saves us
the trouble of juggling prefixes manually.
