application: new 'insert action group' private api

Add a new private API to GtkApplication akin to
gtk_widget_insert_action_group().

We'll use this to insert a few extra actions at the app level with a
separate namespace for the special items in the Mac OS application menu.

https://bugzilla.gnome.org/show_bug.cgi?id=720552

gtk/gtkapplication.c

@@ -1545,6 +1545,14 @@ gtk_application_get_action_muxer (GtkApplication *application)
   return application->priv->muxer;
 }
 
+void
+gtk_application_insert_action_group (GtkApplication *application,
+                                     const gchar    *name,
+                                     GActionGroup   *action_group)
+{
+  gtk_action_muxer_insert (application->priv->muxer, name, action_group);
+}
+
 void
 gtk_application_handle_window_realize (GtkApplication *application,
                                        GtkWindow      *window)