WindowManager: Move more of the menu management logic to WSMenuManager

This patch moves a whole lot of the menu logic from WSWindowManager to
its proper home in WSMenuManager.

We also get rid of the "close_current_menu()" concept which was easily
confused in the presence of submenus. All operations should now be
aware of the menu stack instead. (The concept of a single, current menu
made a lot more sense when there were no nested menus.)

Servers/WindowServer/WSClientConnection.cpp

@@ -388,7 +388,7 @@ void WSClientConnection::handle_request(const WSAPIDestroyMenuRequest& request)
         return;
     }
     auto& menu = *(*it).value;
-    WSWindowManager::the().close_menu(menu);
+    menu.close();
     m_menus.remove(it);
     remove_child(menu);
     WSAPI_ServerMessage response;