protected override void OnMouseDoubleClick(MouseButtonEventArgs e)
 {
     var error = e.GetDataContext() as XamlError;
     if (error != null) {
         Shell.Instance.JumpToError(error);
     }
 }
 void Toolbox_DragStarted(object sender, MouseButtonEventArgs e)
 {
     PrepareTool(e.GetDataContext() as ControlNode, true);
 }