Blender Git Loki

BKE Collection: Add new function to rebuild parent relationships.

It's not always possible to keep 'by hand' parent relationships valid in
collections hierarchy. Add functions to remake those
(re-using/factorizing code from `readfile.c` `lib_link_collection_data()`
function).

Can't stress again how painful it is to have those kind of backward
relationships in our data structures, those *always* end up being
serious issues to keep in sync... Should only be generated on the fly
when needed, period. :(