Add `WindowEvent::Mapped`

[colinmarc]

At least on Wayland, it's useful to know when your window is actually presented. In general, this would allow delaying some logic until the "final" initial state of the window is known.

I'd like to suggest a WindowEvent::Mapped, which would fire when the compositor sends wl_surface@enter. I don't know anything about how other platforms work so I can't suggest a mechanism there, but I can't imagine it's all that different.

[MarijnS95]

On Android this might possibly be implemented by the actual onPause and onResume callbacks. Though a NativeWindow on Android is really just a buffer producer which could send its frames anywhere, not necessarily the display compositor (but we know the Activity around it at least).