Implement ParentingTracker. This is a helper utility used by client side Scenes to handle mass imports of Entities (on the client side). The bug was that if there was parenting in the set of entities that were created on the client side, the server changed those IDs and acked them back to the client. But there was nothing on the client to fix broken parenting after the fact. Does not yet take Entity level parenting into account, the proper place for this logic should be examined separately, but does fix broken EC_Placeable::parentRefs. Also XML/binary content loads do not use this utility, left todo comments and it should be checked and tested separately.