Allow React editors to be used as mini-editors with core.useReactMiniEditors option

This is basically working, but the biggest remaining obstacle is determining when React-mini-editors are hidden in a comprehensive way.

* [x] Don't show wrap guide in mini editors
* [x] Don't capture mouse wheel events in mini editors
* [x] Clean up all the DOM measurement sequences
* [x] Display placeholder/initial text correctly in settings view
* [x] Handle editors being mounted in an invisible state
* [x] Fix padding + colors combination
* [ ] Review old editor specs and make sure we cover everything
* [x] Check that styling matches