Determine if we should map top level QWindows to UIWindow or UIWindowScene

A UIWindowScene can have multiple UIWindows

https://developer.apple.com/documentation/uikit/uiwindowscene/3198091-windows?language=objc

It's not entirely clear yet if the semantics of a top level QWindow maps better to a standalone UIWindowScene (with a single UIWindow), or to individual UIWindows inside a single UIWIndowScene (managed by QIOScreen possibly).

 This is something we need to research, and find the API that maps the best to our use-case, both in terms of semantics and the APIs and callbacks we can map to/from.