public void RemoveDecorator(RectangleDecorator decorator)
 {
     decorator.RedrawNeeded -= HandleDecoratorRedrawRequest;
     decorators.Remove(decorator);
 }
 public void AddDecorator(RectangleDecorator decorator)
 {
     decorators.Add(decorator);
     decorator.RedrawNeeded += HandleDecoratorRedrawRequest;
 }