Allow hiding menu bar on macOS when using Fullscreen::Borderless

[madsmtm]

Discussed in #3878

^{Originally posted by TarekAS August 17, 2024}
In some use cases, such as games, we would want the fullscreen mode to take up the entire screen. This includes the menu bar and the dock on macOS. In fact, the official docs recommends BorderlessFullscreen for games, yet does not provide options to disable the menu bar.

So, how can I hide the menu bar when using BorderlessFullscreen mode? This is currently a blocker for bevy.

Discussed on Matrix.