Add undoing & redoing to the editor

It's very easy to make mistakes in VVVVVV's built-in level editor, with
no way to undo them. This commit adds an undo and redo system, bound to
CTRL+Z for undo and CTRL+Y for redo. The undo and redo stacks don't
have any limits, but could easily happen in the future. VVVVVV's data
is small enough where this should never be an issue, however.

No notes show up for undoing and redoing, because keeping track of what
specific action you're doing may bloat the system, and would get
annoying for the end-user. Notes are a bit annoying in general, even.