RFC: Set/unset focus to overlay sheet on open/close
This is required to capture the Esc key.
It takes focus from the background scope, and tries to set it back after closing.
There might be some other focus shenanigans (for instance, after this,
forcing focus on the footer in
onSheetOpenChanged doesn't seem to work),
and I'm not 100% sure what's the best item to set focus on.
Follow-up to !14 (merged)