Blender Git Loki

Merge branch 'master' into readfile_factorize_common_id_processing

Merge branch 'master' into readfile_factorize_common_id_processing

Readfile: refactor/factorize more handling of common ID data.

- 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.

Differential Revision: https://developer.blender.org/D6764

NodeTree: Add access to the address of an ID's nodetree pointer.