Blender Git Loki

Fix T50369: Objects can't be deleted from scene when using "link group objects to scene"

Main issue here was that in old usercount system 'user_real' did simply
not allow that kind of thing to work. With new pait of 'USER_EXTRA'
tags, it becomes possible to handle the case correctly, by merely refining
checks about indirectly use objects whene removing them from a scene.

Incidently, found another related bug, 'link group objects to scene' was not
incrementing objects' usercount - bad, very very bad!