Add history and undo to the GUI

[rgleichman]

Child of Issue #11

This can be done by having all user interactions create events which are saved once executed. All actions have an undo action.

[rgleichman]

Further changes required:

• IDs that are unique for the entire duration of the program. This prevents cherry-picked history actions from having conflicting IDs.
• To implement Emacs style undo, we'll need to save the undo position (how far back in history we're undoing)
• The undo user input increments the undo position, all other user inputs sets the undo position to the head of the history.
• Undoing an action adds the inverse of that action to the history.
• (optional for now) Add a "quit" input. Equivalent of C-g in Emacs to reset the undo number.
• (optional) Add a user visible counter of how far the undo position is from the original head of the history.