Purge old undo memory

[meh2481]

When working with very large spritesheets and adding and subsequently deleting animations, the program can run out of allocate-able image memory and images start silently failing to load (show in-editor as empty boxes). Restarting the application fixes the problem. This is probably due to past undo steps maintaining the old images in memory, preventing new image memory from being allocated. There should be a limit to how much memory the sum of all undo steps is allowed to use, and the oldest undo steps should be removed one at a time once this memory limit is exceeded.