Blender Git Loki

LibOverride: Also consider regular embedded IDs as virtual overrides.

Those (aka root node trees and master collections) share same 'traits'
as the shapekeys: RNA cannot assign them, so we need to handle them as
sub-data of their owner IDs.

Not sure how much this will help in improving support of overrides for
scenes and nodetrees, but this is a mandatory step towards that goal for
sure.