[Issue Migration] Add a mode where the goal container has no notches

[lcb931023]

https://github.com/kyung01/xo-fractions-game/issues/24

For the early levels, it makes sense to help the kid along by showing them most clearly what is going on. But in higher levels, sometimes the quickest and easiest way to get a right answer was to guess a combination, and then check the notches on the tank. For this reason, I think it would be beneficial to eventually have a mode like this available, either toggled (as in easy/hard) or something that turns on automatically as you go up levels.