Components: Close popover by escape keypress

[aduth]

Related: #2323

This pull request seeks to enhance the Popover component to handle escape keypress and focus return for closing the focused dialog. This also helps consolidate existing but inconsistently-applied approaches for this behavior.

Testing instructions:

Verify that a focused popover can be closed by clicking outside or pressing escape. When pressing escape, observe that focus returns to the element which had been focused prior to opening the dialog.

1. Navigate to Gutenberg > New Post
2. Click Insert in the header menu
3. Press Escape
4. Note that the Insert button is focused

(Repeat for Visibility, Schedule popovers)

Caveats:

When tabbing through to the end of a popover's tabbable elements, we should do one or the other of:

• Circle back to beginning of tabbables in popover (constrain focus, as in a modal)
• Direct focus to next in sequence from where popover had been mounted

[codecov]

Codecov Report

Merging #2697 into master will increase coverage by 0.02%.
The diff coverage is 41.17%.

@@            Coverage Diff             @@
##           master    #2697      +/-   ##
==========================================
+ Coverage   33.64%   33.67%   +0.02%     
==========================================
  Files         185      185              
  Lines        5584     5591       +7     
  Branches      973      975       +2     
==========================================
+ Hits         1879     1883       +4     
- Misses       3138     3139       +1     
- Partials      567      569       +2

Impacted Files	Coverage Δ
components/popover/detect-outside.js	25% <ø> (ø)	⬆️
editor/inserter/index.js	0% <0%> (ø)	⬆️
editor/inserter/menu.js	49.59% <100%> (+1.18%)	⬆️
editor/sidebar/post-schedule/index.js	66.66% <100%> (+1.66%)	⬆️
editor/sidebar/post-visibility/index.js	45.23% <25%> (-0.92%)	⬇️
components/popover/index.js	87.64% <37.5%> (-5.05%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c6ceb8b...9c539d0. Read the comment docs.