Correctly do undo-redo

[stefnotch]

No need for a tree. Just do https://github.com/zaboople/klonk/blob/master/TheGURQ.md

Aka

• Undo stack (as per usual)
• Redo stack (as per usual)

But with one special tweak

1. Undo puts stuff on the redo stack (as per usual)
2. Now I start editing
3. So I grab the entire redo stack, and put it onto the undo stack (going to the present before the undos)
4. Then I undo all these changes, putting that onto the undo stack (going to the point in time where I am)
5. And now I can put my edits onto the undo stack

Thus, it gives us a linear history that looks like
edits => doing edits backwards => new edits

Emacs does that as well.