Blender Git Loki

Fix T62970: Scene Copy: remove 'linked objects/obdata', add 'linked collections'.

'Linked objects' option was not behaving correctly before, effectively
linking in collections, so this one has been renamed to just 'Linked Copy',
and gives a fully shallow copy of current scene.

'Linked Obdata' was not really useful, kind of confusing, and was
painful to maintain, so dropping it now.