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/util.c

@@ -555,6 +555,12 @@ meta_gravity_to_string (int gravity)
     }
 }
 
+char*
+meta_external_binding_name_for_action (guint keybinding_action)
+{
+  return g_strdup_printf ("external-grab-%u", keybinding_action);
+}
+
 static gboolean
 zenity_supports_option (const char *section, const char *option)
 {