ui/cocoa: Let the platform toggle fullscreen

It allows making the window full screen by clicking full screen button
provided by the platform (the left-top green button) and save some code.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Tested-by: Rene Engel <ReneEngel80@emailn.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-ID: <20240224-cocoa-v12-6-e89f70bdda71@daynix.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>