GUI: implemented new way of deleting groups

- Delete list of groups instead of single groups.
  This will ensure, that delete is performed in right order and
  uses 1 single call to core instead of multiple.
Merge pull request #301 from zlamalp/deleteGroups

GUI: implemented new way of deleting groups

- Delete list of groups instead of single groups. 
  This will ensure, that delete is performed in right order
  and uses 1 single call to core instead of multiple.