Components: Implement focus return for Dropdown component

[aduth]

This pull request seeks to improve the Dropdown component introduced in #2888 to restore the focus return behavior. After #2888, it is no longer possible to escape out of an inserter using the keyboard alone. With these changes, the Dropdown component now handles the Popover's onClose callback and ensures focus is returned to the toggle leveraging the withFocusReturn higher-order component.

Testing instructions:

1. Navigate to Gutenberg > New Post
2. Toggle an inserter
3. Press escape
4. Note that the inserter is closed and focus returned to the toggle

[codecov]

Codecov Report

Merging #2911 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2911      +/-   ##
==========================================
+ Coverage   33.92%   33.93%   +0.01%     
==========================================
  Files         193      193              
  Lines        5701     5702       +1     
  Branches     1000     1000              
==========================================
+ Hits         1934     1935       +1     
  Misses       3187     3187              
  Partials      580      580

Impacted Files	Coverage Δ
components/dropdown/index.js	87.5% <100%> (+0.83%)	⬆️

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[gziolo]

Code LGTM and it tests well.

At first I noticed missing unit test for the use case that is being fixed here. I did some research and discovered that withFocusReturn already has tests which ensure this behavior is working as expected. We may leave it as it is because it seems like an implementation detail.