Grab shapes by a specific point

[mkhan45]

Right now, shapes can be dragged from their center. To do this, a velocity towards the mouse is applied to the center of the shape.

There should be an option to drag shapes from an arbitrary point on them. This way, it would be easier to rotate bodies. AFAIK, nphysics doesn't allow adding a velocity to a specific point on an object, meaning that this has to be done by applying a force at a point. I think that something has to be done to map object local coordinates to global coordinates and vice-versa

[mkhan45]

This should probably wait on #40 because it might be different between nphysics and rapier