Implement inertial pan and possibly bounce on workspace

[tmickel]

Consider Zynga Scroller:
visit on a touch device:
https://zynga.github.io/scroller/demo/dom.html

Once we have an optimized workspace pan, I'd like us to consider using this in Scratch-Blockly. It has a great API that I think would be very quick to integrate in, and would likely make workspace panning "feel" very good (with a bit of animation, inertia, etc., as you expect a panning surface to behave). On a mouse device, we can make the workspace behave exactly as it does now (no bounce, no inertia).

Of course before we put it in we'll need to make the actual workspace pan perform well :)