Move setting of accessible slider attributes to ISLCObjectNode

[jessegreenberg]

For #49. In GravityForceLabBasicsScreenView:

    // a11y - TODO: could move to a common type for inverse square law common sims
    var objectProperties = [ model.object1.positionProperty, model.object1.radiusProperty, model.object2.positionProperty, model.object2.radiusProperty ];
    Property.multilink( objectProperties, function( position1, radius1, position2, radius2 ) {
      mass1Node.setAccessibleAttribute( 'min', model.getObjectMinPosition( model.object1 ) );
      mass1Node.setAccessibleAttribute( 'max', model.getObjectMaxPosition( model.object1 ) );
      mass2Node.setAccessibleAttribute( 'min', model.getObjectMinPosition( model.object2 ) );
      mass2Node.setAccessibleAttribute( 'max', model.getObjectMaxPosition( model.object2 ) );
    } );

I think it would be nice to address this, this will be necessary for the other ISLCObjectNodes. @mbarlow12 would you mind working on this?

[jessegreenberg]

Wait... maybe this can be deleted entirely. In AccessibleSlider.js

          // listeners, must be unlinked in dispose
          var enabledRangeObserver = function( enabledRange ) {

            // a11y - update enabled slider range for AT, required for screen reader events to behave correctly
            self.setAccessibleAttribute( 'min', enabledRange.min );
            self.setAccessibleAttribute( 'max', enabledRange.max );
          };
          this._enabledRangeProperty.link( enabledRangeObserver );

@mbarlow12 can you see if the code in GravityForceLabBasicsScreenView can safely be removed and make sure the accessible attributes are being set correctly in the a11y slider trait?