Cleanup of Ragdoll API

SkeletonModel also derived from Ragdoll, which required a bunch of extra "ragdoll" qualifiers in the Ragdoll class API to prevent namespace confusion.  This and the fact that the SkeletonModel was being added to the PhysicsSimulation twice in two different data structures was was getting annoying... so I cleaned it up.

SkeletonModel now HAS a SkeletonRagdoll (instead of BEING a Ragdoll).  SkeletonRagdoll is an implementation of the pure virtual Ragdoll API that has a backpointer to its SkeletonModel (so it can have access to SkeletonModel::_jointStates which has elements that correspond directly to elements in Ragdoll::_points).

Other than that nothing has changed.