Revert "Use a child scene node for the accumulation root"

This reverts commit d6f923f2743afd4bd781e5a31bc5f897f3f18785.

We don't need it for any of the NIFs we're currently handling. As long as
there's no NIF files that would break it, we should require a stationary root
if an animation wants to accumulate. If we must, a better idea may be to inject
an extra bone into the skeleton instance and make that the accumulation root.
Use an array to store the entity parts
Fix handling of filtered entities
Revert "Revert "Use a child scene node for the accumulation root""

This reverts commit 376dfed15ba01cee8d24d122dfafc797a86fd303.

I was wrong. It's needed at least for NPCs since they're attaching multiple
animated skeletons to an object, and they all need to be offset correctly.

Would be nice to use a Node, Bone, or TagPoint instead of a hefty SceneNode,
though.
Run an aniamtion update after "playing" the inventory idle

This is so all the NPC parts get updated correctly.