Ragdoll Part 2: make MyAvatar's skeleton's collision shapes follow ragdoll

The RagDoll knows how to update a collection of shapes, however the simulation loop is not complete: we can't influence the ragdoll by colliding with MyAvatar's skeleton shapes against the world or other avatars.

I didn't want to break any PrioVR + Faceshift hug demos, so the experimental code is only enabled when the Developer --> Avatar Options --> Collide As RagDoll menu item is checked.  When this is done AND you enable Developer --> Avatar Options --> Skeleton Collision Shapes then you will see the skeleton's collision shapes being updated by the RagDoll simulation.  The RagDoll slaving has been deliberately slowed down so that the lag between the avatar movement and the RagDoll is easily visible.
The base Model::updateShapePositions() method has been overridden for the derived SkeletonModel class so that it can optionally use the RagDoll.