Undo/redo system

[MrStevns]

Derived from #953, this PR seeks to integrate as little as possible, so that we can focus on the manager and element classes and then merge those when we find the manager ready.

To ensure even greater success with this it would also be a good idea to write tests for each backup element, so that we can verify that the implementations work as expected.

I have not verified that the backup logic still works but considering I haven't touched it in a long time and it worked the last time I worked on it.. I would expect it to still work.