Kinematic demos

Just the one wobbly platform demo. Really rudimentary mouse-control. The important bit is that the kinematic body is moved by assigning velocity only, so it produces correct contact response. 

I am not sure how to make the view default to being more zoomed out. That would make this demo nicer.

If you'd rather this be a separate demo from the original one (with the box going up and down at +/-5 z-velocity) let me know