Added rudimentary undo/redo functionality

[Cruix]

Closes #38

Added undo and redo for the following operations:

• Placing an atom/atoms
• Deleting an atom/atoms
• Editing the variables of an atom
• Moving an atom to the top/bottom of the tile
• Moving a selection
• Deleting a selection
• Pasting a selection

At the moment, moving a selection is considered two operations: pasting the selection to the new place, and removing it from the old place.

The only way to use undo and redo is to use the dropdown menu. No hotkeys yet.

[ghost]

you are a god.

[Cruix]

You mean keeping a copy of the entire map for every single change that is made? That seems a bit excessive.

[monster860]

No, keeping only the changes to the map (location (1,1) used to point to "aa" now it points to "bb") and the meanings of the keys referenced before and after the change.

[bauen1]

Sure, it uses a bit of extra memory but it's not like we don't have a fuckton of it

And thats how bloatware happens.

Keeping a copy of the entire map will make its a memory gun and make it slow, how about a diff for each action instead ?

[monster860]

how about a diff for each action instead ?

That is literally what I've been suggesting the whole time.....

[Iamgoofball]

@monster860 @Cruix can you guys do this

[monster860]

I don't like the way this change is implemented.