Add `aftertoggle` event for popover

This CL adds an async `aftertoggle` event to both the show and hide
popover transitions, and renames the event class from BeforeToggle
to PopoverToggle, to be used by both events.

This was resolved by OpenUI here:
https://github.com/openui/open-ui/issues/342#issuecomment-1380882740

Bug: 1307772
Change-Id: I996be74b90f43eee4bf859cecfed051fa6f633d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4160443
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1092158}