Account for safe area insets

[bbrk24]

No idea if this is even a thing in backends other than UIKit but it should be possible to say what goes into the safe area inset vs what doesn't.

This will also allow split views to be supported on Apple TV and is a prerequisite for supporting them on iPhone. (How to handle narrow screens like portrait-orientation phones is an open question.)

I am open to suggestions for what the API should look like as I don't have (m)any ideas myself.