Refactor GeoFeatureGroup and derive it from DocumentObjectGroup rather than GeoFeatureObject to unify code of those two classes.
This one prevents thurther object structure damage if DocumentObjectGroup::Group connains multiple links to the same feature. Also minorr changes: - Fix some spacing - Ask the user if he wants to delete group content
git-svn-id: https://free-cad.svn.sourceforge.net/svnroot/free-cad/trunk@5000 e8eeb9e2-ec13-0410-a4a9-efa5cf37419d