This change will make handling of liboverrides hierarchies (especially resyncing) much easier and efficient. It should also make it more resilient to 'degenerate' cases, and allow proper support of things like parenting an override to another override of the same linked data (e.g. a override character parented to another override of the same character). NOTE: this commit only implements minimal changes to add that data and generate it for existing files on load. Actual refactor of resync code to take advantage of this new info will happen separately.