Tilt sensitivity

In this PR:

Added a back button.
Added a tilt sensitivity global variable to track user sensitivity.
added visual indicator of selection on sensitivity menu.

Notes:

There is some code in here that needs refactoring, but in the interest of time will be done in a future commit.
I think the ball should appear after the options are done fading in. The user probably has their phone tilted slightly, which means that the ball ends up falling to a corner before the options are visible, so it's less obvious that they are getting live feedback.