Reuse the existing infrastructure in GtkActionMuxer for propagation of
accelerator information: in particular, what accel label ought to appear
on menu items for a particular action and target.

This is a good idea because we want accels to travel along with the
actions that they're tied to and reusing GtkActionMuxer will allow us to
do that without creating another hierarchy of a different class for the
sole purpose of filling in accel labels on menu items.

Doing it this ways also allows those who copy/paste GtkActionMuxer to
insert the accels for themselves.

Add a new method on the GtkActionObserver interface to report changes.

This patch introduces a new concept: "action and target" notation for
actions.  This format looks like so:

  "'target'|app.action"

or for non-targeted actions:

  "|app.action"

and it is used over a number of possible alternative formats for some
good reasons:

 - it's very easy to get a nul-terminated action name out of this format
   w...