Editor: Allow undo / redo

[vHanda]

When editing a note it's essential to be able to undo some changes that have been made.

Notes to self: Use https://github.com/jheyne/diff-match-patch to compute a patch after each change in Editor, and keep storing the patches. The undo, redo buttons can be used to apply these patches.

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[akvadrako]

I would especially like to be able to undo deletions, or just in general revert to previous commits on a file/dir basis.

[vHanda]

I've released a very basic undo / redo support for the Raw Editor. (Version 1.74)

[vHanda]

Fixed with 1.82.2