Simplify popover=hint behaviors

- Showing a hint popover doesn't hide auto popovers.
- Showing a hint popover hides non-parent hint popovers.
- Clicking outside popovers consistently hides them.
- Hiding an auto popover does not hide unrelated hint popovers.
- Showing an auto popover as a child of a hint popover downgrades the auto popover to a hint.
- Showing a popover during the show or hide of another popover is no longer allowed.

Tests: web-platform-tests/wpt#59237.

Fixes #12304, fixes #12346 fixes #12355, and fixes #12356.

Author: jakearchibald