Blender Git Loki

Fix T75680: Nodegroup user count increased when file saved in edit group mode.

This editor's code was a bit schizophrenic, some parts considering its
nodetree usages as real refcounted ones, others, as shallow 'user one'
ones...

Editors should not be real ID users anyway, unless there are *very* good
reasons for it, so swich it to fully 'shallow' usage now.