• 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…