EuiFormRow with nested EuiPopovers using ownFocus

[thompsongl]

Summary

Lots of manual testing welcome

Discussed primarily in #eui Slack, the issue as raised by @legrego shows serious performance problems resulting from DOM focus updates in EuiPopover.

Causes

1. React portal component subtrees vs. real DOM position: we rely on .contains() to prevent unnecessary focus updates, which simply doesn't cut it when portals get involved.
2. EuiPopover is likely too eager to manually change focus during its componentDidUpdate method.

Solution

No. 1 is a real thing we (and every other app in the React ecosystem) need to solve. The best answer is likely to let React fix it with the proposed FocusManager API.

We have full control over No. 2, so that's where to start.

Events where updating focus should happen

Event	Update focus?	Why
open	✅	We've change active elements and users have entered an altered flow/tab order
close	❌	EuiFocusTrap handles this case and returns focus to the last active element
scroll	❌	If no click event happens, focus has not changed. If a click is involved, EuiFocusTrap has already (correctly) removed focus from the popover
reposition	❌	Same as 'scroll'
blur	❌	Same as 'scroll'
prop updates	❌	initialFocus and ownFocus are the only props relevant to focus. Neither should alter an already open popover. They will update on subsequent open calls.

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode
- [ ] Any props added have proper autodocs

• Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately

- [ ] Jest tests were updated or added to match the most common scenarios

• This was checked against keyboard-only and screenreader scenarios

- [ ] This required updates to Framer X components

[chandlerprall]

Changes LGTM; tested locally, not seeing performance issues from the focus fight

[legrego]

Thanks so much for the fast turnaround on this! 🎉