Content engine: expand only required groups during loading (ported from /branches/groupstorage)

git-svn-id: http://svn.flexive.org/repository/flexive/trunk@2890 d35d4baa-7912-0410-b0ae-edacad209e15
Content engine: merged rest of group position fixes from /branches/groupstorage

git-svn-id: http://svn.flexive.org/repository/flexive/trunk@2891 d35d4baa-7912-0410-b0ae-edacad209e15