Fast Pinch-Zoom-Pan support on the canvas sizer

This patch adds:

A new class ZoomHelper that is used to control the zooming of the document
by API and/or touch gestures. CSS3D transforms are used to do extremely fast bitmap scaling
of the sizer.
1-finger Panning is actually native scrolling so that you can see the scrollbars. Pinch-Zoom-Pan is smoother and non-native.

The zoom slider, being very unweildy on touchscreens, is disabled when touch support is present.

This PR show custom scrollbars in touch mode. This is required because webkit on mobile does not seem to show any scrollbars if `-webkit-overflow-scrolling: touch` is not enabled -- and we cannot enable that because it's kinetic scrolling is not controllable or trackable, due to it not firing any events. 
They are only visible when actually scrolling, to mimic iOS and similar other touch platforms.