// For changing the current state of the diagram.
 public MoveShapeCommand(Shape _shape, double _offsetX, double _offsetY)
 {
     shape = _shape;
     offsetX = _offsetX;
     offsetY = _offsetY;
 }
 // For changing the current state of the diagram.
 public AddShapeCommand(ObservableCollection<Shape> _shapes, Shape _shape)
 {
     shapes = _shapes;
     shape = _shape;
 }