Previously, GtkWindow would add the "app" action group to its own
toplevel muxer.

Change the setup so that GtkApplication creates the toplevel muxer and
adds itself to it as "app".  Use this muxer as the parent muxer of any
GtkWindow associated with the application.

This saves a small amount of memory and will allow for accels to be
propagated from the application through to all of the windows.