Optimise undo system

[NicDoesCode]

It would be good to avoid writing the entire project into undo memory every time a change is made, but instead make a list of things such as the action taken, and the tiles that were updated.

That will save memory and may make undoing and redoing a lot quicker, as when the undo occurs we can make a decision on whether the entire image needs to be redrawn, or just a specific tile.

This will require a lot of work but will have a great payoff.