Duplicated numbering of Object labels

[pixelzoom]

For #374

Duplication in ArrowObjectSceneLabelsNode and LightObjectSceneLabelsNode:

    options.isNumberedProperty.link( ( isNumbered: boolean ) => {
      if ( isNumbered ) {

        // Object N
        this.setText( StringUtils.fillIn( geometricOpticsStrings.label.objectN, {
          objectNumber: lightObject.opticalObjectNumber
        } ) );
      }
      else {

        // Object
        this.setText( geometricOpticsStrings.label.object );
      }
    } );

[pixelzoom]

Done in above commits. Also addressed duplication related to numbering and real/virtual in image labels.