Add a reset sprite button for exercising

Hi I want to add a reset sprite button for those sprite exercises. It comes from my own frustration when I messed up the sprites. By the way, why was the `show answer` button hidden for these exercises?

Another thing I think should be added is: dispose the subscriptions of the previous 'run', when the user changes its source code. This is the root cause of many buggy behaviour I've seen. Any opinion on that?