Generalized undo system

[NathanWolf]

I want to port the undo ("rewind") system from Spells over to Gameplay.

I have a few features I want to add, and some performance improvements to make- mainly giving each BlockList a BoundingBox.

Otherwise, for starters at least, I'm basically just moving the system over wholesale.

[NathanWolf]

Oh, also, make it persistable. Did I forget to mention that?

We'll see how this goes- going to be the first real test of Persistence at any kind of scale.

It may need to wait for async auto-save, or player login/logout might be a little laggy.