Create GestureManager. VerticalScrollGesture used in ScrollView
Gestures: Add TapGesture and demos/tapgesture.html
Gestures: Add mouse support.
Gestures: Add mousewheel support to the GestureManager
Gestures: Use pageX/Y instead of hacky offsetX/Y.

Our offsetX/Y hack using getClientBoundingRect() is no good. It's
unstable as elements move around, and made the drag gesture jittery.

pageX and pageY seem to have exactly the semantics we want: relative to
the document, including whatever scrolling has happened.
Gestures: Add pinch/stretch/twist gesture.
Gestures: Don't call addPoint if the gesture doesn't define it.
Gestures: Scroll gesture handles horizontal too. Add demo.