Selective undo

[scofalik]

Let's implement a selective undo. Using our OT implementation it's possible to undo some past changes (meaning – not the most current one but some previous ones).

[scofalik]

Selective undo proof-of-concept has been implemented, you can see it in action here: https://twitter.com/scofalik/status/740204196977385474

The changes in the feature are mostly UI things. Of course the heavy stuff is in Undo feature, which is one of core CKEditor5 features and I've been working a lot to fix bugs there.

Check out ckeditor5-engine branch hackathon/selective-undo and ckeditor5-undo branch hackathon/selective-undo to see some of those changes (other changes are already on master).

There are a few issues in undo mechanism to work on, though, like:

• maybe splitting History into "real" history (the deltas that were applied) and "compressed" history (stuff that is processed so the undo works well). Now it is a bit messy since it is together in one class.
• clean up mess with batch types - this subject is not organized at all (it already has issue in ckeditor5-engine).
• fixing some problems with redo and selective undo
• fixing some buggy cases with undo

But it's going well!

[scofalik]

I've pushed some code that is breaking selective undo for a while, but will fix it tomorrow morning :)