macos: select new key window after processing events

If we closed a key window in response to events, we need to denote another
window as the new key window. This is easiest to do from an idle so that
we don't clobber notification pairs of "did resign"/"did become" key
window.

We have a sorted set of surfaces by display server stacking, so we can
take the first one we come across that is already mapped and re-show it
to become key/main.