public void SelectionChanged(DiagramToolElement toolElement)
 {
     foreach (DiagramToolElement tool in tools) {
         if (toolElement != tool) {
             tool.active = false;
         }
     }
 }
 public void Add(DiagramToolElement  element)
 {
     if (defaultTool == null) {
         defaultTool = element;
     }
     tools.Add (element);
 }