Add compositor hook to process keybindings selectively

Currently keybindings are blocked while the compositor holds a grab; if
we want a keybinding to be available anyway, we use captured ClutterEvents
to determine the KeyBindingAction the event would have triggered and
run our own handlers (ugh).
Instead, provide a hook to allow the compositor to filter out keybindings
before processing them normally, regardless of whether the compositor
holds a grab or not.

https://bugzilla.gnome.org/show_bug.cgi?id=688202