Blender Git Loki

Fix (unreported) broken collections after undo/redo, or remapping (leading to crashes).

Those are two cases where keeping infamous backward `parents` pointers
of collections in sync is kind of impossible to do... So rebuilding
those relationships from scratch instead.

Fixes e.g. a crash when undoing, then reloading a library, and likely
many more weird ones like that.

Uncovered while investigating T64764.