WindowServer: Move menu related code from WindowManager to MenuManager

Menus are now owned by menu manager instead of being split between the
window manager and menu manager. If the window server wants to change
a menu, or call menu related functionality, this will need to be done
through the menu manager.

Further refactoring is likely needed, but this seems like a good start
for seperating menu logic from window logic.

Servers/WindowServer/WSClientConnection.cpp

@@ -85,7 +85,7 @@ OwnPtr<WindowServer::DestroyMenubarResponse> WSClientConnection::handle(const Wi
         return nullptr;
     }
     auto& menubar = *(*it).value;
-    WSWindowManager::the().close_menubar(menubar);
+    WSWindowManager::the().menu_manager().close_menubar(menubar);
     m_menubars.remove(it);
     return make<WindowServer::DestroyMenubarResponse>();
 }