The current process of implementing GActionObserver is annoying and the
GSimpleActionObserver interface leaves a lot to be desired.  Introduce a
new class, GtkActionHelper that gives you pretty much everything you'd
want to do as an implementor of GtkActionable.

The GtkActionHelper also features an "application" mode that is not
associated with a particular GtkWidget but rather with whatever widget
happens to be the active window of the given GtkApplication at a
particular point in time.  This will be useful for the Mac OS menubar.