Add undo functionality (memento)

[Neop]

Yeah, blame me, I actually forgot this when I first planned MUD Map 2 >.<

[avatard]

I'd like to give this a shot if you aren't working on it - unless there's not really a need for it?

[Neop]

I'm currently working on this, I'll push the branch once it's more stable.
I decided to do some kind of memento pattern that aggregates the changes of Place and Layer to World, which is a bit tricky but I think it will work out.