Basic save states

Board states can be saved and restored. Saved states are basically
pointers into the undo queue. Saving a state pushes the current undo
queue's size. Restoring a state pops the saved state stack and
repeatedly calls undo() until the undo queue's size equals the popped
size. Passing a save state through undo() deletes that saved state.

Currently, these actions are triggered by Ctrl + L and Ctrl + S.

Also, the current implementation very inefficiently recalculates the box
count and streaks after every single undo().