Duplicated positionProperty listener

[pixelzoom]

For #374

LensNode, MirrorNode, and ProjectionScreenNode all have this listener:

    lens.positionProperty.link( position => {
      this.translation = modelViewTransform.modelToViewPosition( position );
    } );

Are there others? Should I factor out a "positionable" model element and Node subclass? It might be more trouble than it's worth.

[pixelzoom]

Investigated... This is definitely not worth the trouble. Closing.