mms152_ts: support for gesture-mode, and expand gesture functionality

- when a touchkey is pressed, if there is a screen touch within x ms,
  gesture mode will be enabled and the touchscreen driver will become
  gesture-only, and won't report touch events to the OS. all fingers
  up will reset it.

- added support for TKEYS flag in gesture definitions. any gesture
  with this flag will require a touchkey to be held down while it
  is being drawn.

- added support for EXCLUSIVE flag in gesture definitions. normal gestures
  are run as soon as their hotspots are triggered, even before fingers-up.
  the problem is if you have a very simple gesture, it will always get
  triggered before you can complete a more complex one. any gesture with
  this flag will instead be run after all fingers are up, presumably when
  the gesture has been completed. if at that point no other gestures were
  triggered, then the gesture event will be broadcast.