make main ragdoll and entity special

also addded some logic (unused) to add ragdolls of other avatars
namechange _frame --> _frameCount
MyAvatar's ragdoll is the main simulation ragdoll
ragdoll simulation now in model-relative frame

(same rotation as world-frame, but translated to MyAvatar origin)
add other ragdolls to simulation
VerletPoint::_mass is now private

We set the mass of other avatars artificially high
so they are less movable.
merge upstream/master into andrew/ragdoll
remove extraneous roots from Model::_jointStates
Merge pull request #3263 from AndrewMeadows/ragdoll

Ragdoll Part 9: