[overlay] keyboard event dispatcher

[willshowell]

Bug, feature request, or proposal:

Feature

Motivation

If focus is not one of an overlay component's focusable elements, keyboard events will land on the body and context is loss for which overlay component should have been targeted.

More specifically, something like this will later enable dialog users to respond to targeted escape events with only a DialofRef instance. See #3460 (comment) and preceding 4-5 comments.

It feels like there needs to be a more involved keyboard event dispatcher for overlays. Probably one listener on the body that dispatches the event to the appropriate overlay based on some combination of event origin and open order (maybe with a separate priority mechanism).

Reproduction of keydown target issue

https://plnkr.co/edit/puYeeRCta8A7moZOPRWI?p=preview

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