To ensure the servoshell window is rendered in fullscreen in current active screen, we need to get the current monitor
on runtime every time set_fullscreen is run. However, currently it uses the 'monitor' field of HeadedWindow struct which was just defined once at app startup. This prevents servoshell running on fullscreen on other monitors apart from the one at start. This change removes the monitor field from HeadedWindow and uses Window.current_monitor() in set_fullscreen function to get the current monitor at runtime and then display window on fullscreen on that monitor.

Testing: Although I am not fully certain, since this is just a small fix for servoshell, it doesn't require any additional testing methods since there aren't any tests for servoshell.

Fixes: #45488