keybindings: Add external grab API

During compositor grabs, all global keybindings that don't go
through mutter's keybinding system are blocked. To allow other
processes to make use of it, gnome-shell will expose a simple
grab API on DBus; for this, add API to grab key combos directly
instead of parsing accelerators stored in GSettings.

https://bugzilla.gnome.org/show_bug.cgi?id=643111

src/core/display-private.h

@@ -460,6 +460,9 @@ void meta_display_queue_autoraise_callback  (MetaDisplay *display,
 void meta_display_remove_autoraise_callback (MetaDisplay *display);
 
 void meta_display_overlay_key_activate (MetaDisplay *display);
+void meta_display_accelerator_activate (MetaDisplay *display,
+                                        guint        action,
+                                        guint        deviceid);
 
 /* In above-tab-keycode.c */
 guint meta_display_get_above_tab_keycode (MetaDisplay *display);