Blender Git Loki

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.

Note that this actually also fixes some bugs (at least, potential ones),
like e.g. missing call to expand_id() for our beloved 'private ID'
(nodetrees & co), in current master code...

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