#34 Undo does not undo setting number correctly

[Dale King]

When you set a square to a number undo does not undo that. You have to do more undos to undo it.

For example:
- Set current number to 1 for instance
- Click and hold to set a pencil mark for a square
- Tap another square to mark that square with the number
- Hit undo and nothing happens
- Hit undo again and then both changes are undone

This is very frustrating when you meant to do a a tap and hold, but it only recognizes it as a tap.