Blender Git Loki

Fix T61683: Linking the instance of a collection crashes Blender.

Do not instance linked object immediately in scene, this was never a
good idea and is doomed to fail nowadays, with complex relations between
objects, collections and scenes.

Instead, this commit refactors a bit linking code to add loose objects
to current scene *after* everything has been imported, and ID pointers
have been properly remapped to new ones - i.e. once new linked data is
supposed to be fully valid, just like we were already doing with
collections.

As a bonus, it means we do not have to pass around scene, view3d etc. to
`BLO_library_link_named_part_ex()` and co.