Components: Trigger focus return by tracking focus events

[aduth]

Partially addresses #2306

This pull request seeks to refactor the withFocusReturn higher-order component to track an instance property reflecting whether the rendered element currently has focus. This proves to be more reliable when the disposable component is portaled to another location in the DOM (#2306), since the previous element.contains test will not match said content. Instead, the implementation here leverages event bubbling (and event bubbling of portaled content) to track at all times whether the wrapped component has focus. Then, the focus return condition is simply a matter of: Is the component unmounting while it has focus?

Testing instructions:

Verify that focus is returned to initial active element after dismissing a withFocusReturn component:

1. Navigate to Gutenberg > New Post
2. Open an inserter
3. Press Escape
4. Note that focus is returned to the inserter button

[codecov]

Codecov Report

Merging #2321 into master will increase coverage by 0.07%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #2321      +/-   ##
==========================================
+ Coverage   25.34%   25.42%   +0.07%     
==========================================
  Files         152      152              
  Lines        4754     4760       +6     
  Branches      807      808       +1     
==========================================
+ Hits         1205     1210       +5     
- Misses       2996     2997       +1     
  Partials      553      553

Impacted Files	Coverage Δ
components/higher-order/with-focus-return/index.js	91.66% <90%> (-8.34%)	⬇️

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[afercia]

Tested on the Inserter, works great. Thanks!