Add a "prev" and "next" button that inc/dec nextFrame by 1 and renders it

[turbomaze]

• essential for users since it helps them see patterns

[turbomaze]

Right now there's:

"watch" --> display the obstacle sequence
"run" --> run the code against the obstacle sequence
"reset" --> go back to the initial state, interrupting a "watch" or "run"
"submit" --> nothing yet

We'd also like to have (in order of priority):
"previous" --> go to the previous frame of the obstacle sequence (this is a "watch", not a "run")
"next" --> go to the next " "
"pause" --> pauses the watch/run
"play" --> resumes the watch/run