Stubbed in animdata-adding for "Armature" data

There are still many todo's to be fixed up here for nice instancing,
but those require some other changes (aka proper proxies first). In the
meantime, we just want to be able to port this all up...
Stub functions for building subgraphs from groups
Compositing nodetree graph building - just plain wrapper for now
Blurb explaining design decisions for how rig graphs are constructed