Add support for trackpad gestures

[jyoshimi]

As far as I can tell there is currently no way in Piccolo to provide support for zooming via pinch gestures at the trackpad. We do get some of this via PMouseWheelZoomEventHandler (I think because track pads emulate the mouse wheel), but the behaviors are not quite as expected. On Mac the two finger swipe zooms, and on PC the zoom is reversed from what would be expected. Thanks.