Implement shift + knob fine-tune mode

[thomasjwebb]

It will need to keep track of the most recently-known knob position, then further changes would be relative, not absolute. The biggest challenge is we'll probably need to keep track of whatever the current value of the thing being changed is but not allow that to create a feedback loop. So let's say the knobMode is Volume and the value of the knob is currently 56. Hitting shift, then turning it up to 70 should change the volume to currentVolume + (70-56)/fineTuneDivider with fineTuneDivider being some number greater than 127.