meta_plugin_begin end_modal are gone from mutter.  wm.push_modal and
wm.pop_modal have to be implemented in gala, say via.

- lib/WindowManager.vala: (interface WindowManager): new field public
Clutter.Grab grab

- src/WindowManager.vala:
   begin_modal(0, time) =>
     proxy.grab = stage.grab ((Clutter.Actor) stage);
   end_modal (display.get_current_time ()); =>
     proxy.grab.dismiss();