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.