Related: #2771

This pull request seeks to assign attributes of the Autocomplete cloned input as a combobox, bound as "owning" the Popover which it controls. 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.

These changes are extracted from commits originally present in #2771, in order to simplify review there to the minimal set of changes to fix the regression introduced to autocomplete behavior after #2323.

Testing instructions:

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.