- Move handling of private ID data (nodetree and master_collection)
under generic ID code. This shortens code a bit, but mostly avoids having
to modify all type-specific callback functions if/when we have to add
generic processing to IDs there.
- Seriously factorize `expand_xxx` area, in the same way we were already
doing in `direct_link_xxx` and `lib_link_xxx` areas.