Add undo/redo

[ambrosiogabe]

Come up with a method for undo/redo. It should undo stuff in a way that makes sense to. For example:

• If I move a timeline clip, undo to the last place it was before the move started
• If I move an object, undo to the last position it was before it started
• If I undo an object creation, delete the object and all it's sub-objects

Etc. Redo should likewise redo things in a sane fashion :)