Inserter: Fix default block slash inserter

[aduth]

Regression introduced in #2323
Related: #2630

This pull request seeks to resolve an issue where the slash inserter introduced in #2630 stopped working after #2323. The issue is that because focus is now moved into a Popover when it transitions from a closed to open state, and because the Autocomplete field closes the popover when it loses focus, the popover is shown but immediately dismissed.

The changes here resolve this issue by adding a new focusOnOpen (default: true) prop to the Popover component to suppress the default focus behavior in cases such as Autocomplete where the focus behavior is not desirable, since it manages keyboard interactivity of the popover controls via the Editable input.

(Note: The following changes were extracted as a separate pull request to simplify review of the minimal set of changes necessary to resolve the regressed behavior. See #2789)

To better represent the relation between the Editable and Popover search listing, props applied to the Editable have been improved using reference from the combobox role specification.

However, this highlighted an issue: Applying arbitrary props to the Editable contenteditable node is not currently possible. In a few cases, we have hard-coded support for a few whitelisted props (style, className, label). Instead, the changes here refactor Editable and TinyMCE to support arbitrary prop assignment, using a "mirror" node which is leverages React reconciliation in the parent Editable component. Since we disable reconciliation for the TinyMCE component, this mirror node serves to copy attributes when changed using MutationObserver. This is made more challenging by the fact that Editable accepts many props, most of which are handled by the root Editable component and should not be assigned to the contenteditable node. While I otherwise discourage propTypes assignments, the usage here helps simplify picking props to assign to the mirroring node.

Testing instructions:

Repeat testing instructions from #2630

Verify that there are no regressions in:

• Attribute assignment to TinyMCE (e.g. try applying background color to Paragraph block, which assigns itself as a combination of class and style attribute)
• Placeholder behavior for Editable

Verify that correct ARIA expanded / ownership / active descendant attributes are assigned to both the mirror node (aria-hidden) and the contenteditable while using the default block slash inserter.

[codecov]

Codecov Report

Merging #2771 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2771      +/-   ##
==========================================
+ Coverage   33.72%   33.77%   +0.04%     
==========================================
  Files         189      189              
  Lines        5636     5640       +4     
  Branches      980      982       +2     
==========================================
+ Hits         1901     1905       +4     
  Misses       3163     3163              
  Partials      572      572

Impacted Files	Coverage Δ
components/tooltip/index.js	83.87% <ø> (ø)	⬆️
components/autocomplete/index.js	89.18% <100%> (+0.14%)	⬆️
components/popover/index.js	88.04% <100%> (+0.4%)	⬆️

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