Inserter: Render menu as content of Popover

[aduth]

This pull request seeks to resolve an issue where clicking the Inserter button in content causes the menu to appear overlapping the button, rather than above it. The issue is described at #2323 (comment):

Seems the div wrapper introduced in #2321 is causing some issues with Popover being able to calculate its offset position. I think eventually we'll want to move withFocusReturn to be part of Popover itself, which should hopefully provide a better long-term solution.

This pull request merely extracts most of the 26b75c8 commit that was already slated for #2323 while other requested changes are in-progress there.

Before	After

Testing instructions:

This is most obvious for inserter menu which opens upwards, meaning it must be for a post which has enough content to allow vertical upward space to open. Try the Demo content or a new post with at least a couple blocks added.

Verify that the content inserter menu opens upward above button:

1. Navigate to Gutenberg > Demo
2. Scroll to bottom of content
3. Toggle Inserter
4. Note insert shows above button

[codecov]

Codecov Report

Merging #2364 into master will decrease coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2364      +/-   ##
==========================================
- Coverage   25.69%   25.66%   -0.03%     
==========================================
  Files         154      155       +1     
  Lines        4811     4840      +29     
  Branches      810      814       +4     
==========================================
+ Hits         1236     1242       +6     
- Misses       3020     3040      +20     
- Partials      555      558       +3

Impacted Files	Coverage Δ
editor/inserter/index.js	0% <0%> (ø)	⬆️
editor/inserter/menu.js	48.41% <100%> (ø)	⬆️
editor/index.js	0% <0%> (ø)	⬆️
editor/modes/visual-editor/block.js	0% <0%> (ø)	⬆️
components/popover/provider.js	100% <0%> (ø)
components/popover/index.js	95% <0%> (+0.26%)	⬆️

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