WindowServer+LibGUI: Make much of menu construction asynchronous

Creating a menu/menubar needs to be synchronous because we need the
ID from the response, but adding stuff *to* menus (and adding menus
to menubars, and menubars to windows) can all be asynchronous.

This dramatically reduces the amount of IPC ping-pong played by
each GUI application during startup.

I measured how long it takes TextEditor to enter the main event loop
and it's over 10% faster here. (Down from ~86ms to ~74ms)

Userland/Libraries/LibGUI/Menubar.cpp

@@ -49,7 +49,7 @@ void Menubar::notify_added_to_window(Badge<Window>)
     for (auto& menu : m_menus) {
         int menu_id = menu.realize_menu();
         VERIFY(menu_id != -1);
-        WindowServerConnection::the().add_menu_to_menubar(m_menubar_id, menu_id);
+        WindowServerConnection::the().async_add_menu_to_menubar(m_menubar_id, menu_id);
     }
 }