cyapa - Introduce better three-finger button emulation
* Change the three-finger button emulation such that the third finger that
you use to tap will emulate the LEFT, MIDDLE, or RIGHT buttons depending
on its X position relative to the other two fingers that are being held
down.
e.g. if you arrange fingers left-to-right A, B, C, and you hold B and C
on the pad and tap A, that's a LEFT button. If you hold A and C and
tap B, that's a middle button, and if you hold A and B and tap C, that's
a right button.
* This methodology allows all three buttons to be emulated cleanly without
having to depress the trackpad's physical button.
* Document all jestures.