Handle actor component caches outside of `Level`

[fingoltin]

When an actor gets a component added or removed, the level responds by updating the component cache in its ActorStorage. There are times when an actor is in a different ActorStorage and we still want to update its component cache (inventory is the big one), so the proposal is for Level to extend or implement an interface that's shared with ActorStorage. Actors would hold a reference to this.