Duplicated drag function

[pixelzoom]

For #374

This is duplicated in 4 draggable Nodes. Can it be factored out? Is it worth it?

    // Drag action that is common to DragListener and KeyboardDragListener
    const drag = () => {
      wasDraggedProperty.value = true;
    };

[pixelzoom]

Two of these occurrences where in subclasses of OpticalObjectNode, so those were moved into that base class.

The other two occurrences are in SecondPointNode and ProjectionScreenNode, both of which extend Node. I decided to leave those alone.

Closing.