Consider moving visibility Properties to the view

[pixelzoom]

For code review #41 ...

These Properties are currently in IntroModel.ts:

    this.isShowingPredictMeanProperty = new BooleanProperty( false, {
      tandem: options.tandem.createTandem( 'isShowingPredictMeanProperty' )
    } );
    this.isShowingMeanProperty = new BooleanProperty( false, {
      tandem: options.tandem.createTandem( 'isShowingMeanProperty' )
    } );
    this.isShowingTickMarksProperty = new BooleanProperty( false, {
      tandem: options.tandem.createTandem( 'isShowingTickMarksProperty' )
    } );

They have nothing to do with the model. And more importantly, PhET-iO clients may expect them to be in the view. So consider moving them to IntroScreenView. For an example, see geometric-options.VisibleProperties.

[marlitas]

Visibility properties have been moved to the view. Thanks for the suggestion!