Very simple class for swipe and pinch(zoom) gesturing. Swipe can also
generate "fling" given a certain velocity threshold. Customers will write
their own detectors or use external packages, but the basics of capturing
swipe and pinch common to most implementations are covered with the basic
GestureControl callbacks used here. Note that swipe is sometimes implemented
with the scale arena when there is only one point (i.e. not two fingers).
