RichText: completely controlled

[ellatrix]

Description

Splitting this out from #14640.

This makes RichText a completely controlled component: changes in props or state are always reconciled with the DOM.

This eliminates a lot of logic trying to figure out of the DOM should be reconciled or not. It's easier to just do it: the DOM won't be touched if no changes need to be made.

How has this been tested?

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[ellatrix]

Expecting one e2e test to fail. Let's see if there are more.

[ellatrix]

@atimmer As part of this, I'm rewriting preparing editable tree etc. a bit. I'm avoiding passing arrays and objects as props to RichText as we can check more easily what changed. This also means that we can get rid of the use of memize, which is a concern as we never clear the cache. I'll assume that, if tests pass, everything still works correctly. :)

[ellatrix]

Going to split it into smaller PRs, one of which is ready: #14664.