made the property that indicates whether the thumb is being dragged available to clients of the slider, see https://github.com/phetsims/energy-forms-and-changes/issues/60
made the property that indicates whether the thumb is being dragged available to clients of the slider, see https://github.com/phetsims/energy-forms-and-changes/issues/60

phetsims/sun,phetsims/sun,phetsims/sun