Actually implement opening the app menu

The last commit added support for the "appmenu" button in decorations,
but didn't actually implement it. Add a new MetaWindowMenuType parameter
to the show_window_menu () functions and use it to ask the compositor
to display the app menu when the new button is activated.

https://bugzilla.gnome.org/show_bug.cgi?id=730752

src/x11/window-x11.c

@@ -2723,7 +2723,7 @@ meta_window_x11_client_message (MetaWindow *window,
       x = event->xclient.data.l[1];
       y = event->xclient.data.l[2];
 
-      meta_window_show_menu (window, x, y);
+      meta_window_show_menu (window, META_WINDOW_MENU_WM, x, y);
     }
 
   return FALSE;