void DeleteEntityHandler(object source, DeleteEntityEventArgs e)
 {
     if (SelectedObjects.Contains(e.entity))
     {
         SelectedObjects.Remove(e.entity);
         NotifyObservers();
     }
 }
 void DeleteEntityHandler( object source, DeleteEntityEventArgs e )
 {
     if( SelectedObjects.Contains( e.entity ) )
     {
         SelectedObjects.Remove( e.entity );
         NotifyObservers();
     }
 }