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/meta/display.h

@@ -139,6 +139,11 @@ guint meta_display_add_keybinding    (MetaDisplay         *display,
 gboolean meta_display_remove_keybinding (MetaDisplay         *display,
                                          const char          *name);
 
+guint    meta_display_grab_accelerator   (MetaDisplay *display,
+                                          const char  *accelerator);
+gboolean meta_display_ungrab_accelerator (MetaDisplay *display,
+                                          guint        action_id);
+
 guint meta_display_get_keybinding_action (MetaDisplay  *display,
                                           unsigned int  keycode,
                                           unsigned long mask);

src/meta/util.h

@@ -93,6 +93,8 @@ guint meta_unsigned_long_hash  (gconstpointer v);
 const char* meta_frame_type_to_string (MetaFrameType type);
 const char* meta_gravity_to_string (int gravity);
 
+char* meta_external_binding_name_for_action (guint keybinding_action);
+
 #include <libintl.h>
 #define _(x) dgettext (GETTEXT_PACKAGE, x)
 #define N_(x) x