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/core/keybindings.c

@@ -2751,7 +2751,7 @@ handle_activate_window_menu (MetaDisplay     *display,
         x += child_rect.width;
 
       y = frame_rect.y + child_rect.y;
-      meta_window_show_menu (display->focus_window, x, y);
+      meta_window_show_menu (display->focus_window, META_WINDOW_MENU_WM, x, y);
     }
 }