gtkwindow: Use actions from focused widget to activate accel

Currently we only take into account the window GActionGroup for
activating the accels.

However, the application could have some custom GActionGroup in the
chain of focused widgets that could want to activate some action if
some accel is activated while that widget is focused.

To allow applications to set accels on widgets that use custom
GActionGroups, simply use the muxer of the focused widget, which
already contains the actions of the parents.

https://bugzilla.gnome.org/show_bug.cgi?id=740682
window: fallback to window muxer if no widget has focus

This ensures that we can still match accelerators for the window even
if no widget currently has focus.