Improve undo behavior

[cxxxr]

[fukamachi]

ref #1034

I also want Lem to keep the cursor position in the undo stack.

[BanchouBoo]

Having the option to make undo history persist between sessions, including branching changes, would be great. In vim I get a lot of use from persistent undo in conjuction with undotree so I can have an interactive explorable history of every change (up to a point) I've made to a file with diffs of each change.

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