Add support for object-level animations

[rdb]

This could be done by adding a joint for each animated object in the animation, but rather than hooking it up to a JointVertexTransform to transform vertices, we would use joint.add_local_transform() or joint.add_net_transform() (ie. the equivalent of actor.exposeJoint) to have Panda copy the transformation to the object in question.

The animated object should probably be made a ModelNode to prevent Panda from flattening it.

Also see panda3d/panda3d#457, which discusses a way of implementing object animations without use of joints in Panda3D.