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!
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