Rework how accels are handled on GtkApplicationWindow.

Instead of having GtkApplication fill the GtkAccelMap which is then used
by GtkApplicationWindow to create a GtkAccelGroup filled with closures
that is then associated with the window, do it directly.

GtkApplication now keeps a list of accels and their actions.
Accelerators on a GtkApplicationWindow ask GtkApplication to execute the
appropriate action.

This saves a fair bit of complexity and memory use (due to not having to
create all those closures and accelmap entries).  The new approach also
supports multiple accels per action (although there is not yet a public
API for it).

This patch (and the ones before) Reviewed and ACK'd by Matthias Clasen.