expose DragListeners for Slider thumb and track

[pixelzoom]

For phetsims/fourier-making-waves#31, we need to know the state of the DragListeners associated with a Slider's thumb and track -- specifically their isHighlightedProperty value. Slider adds the DragListener to the thumb, SliderThumb adds its own DragListener. These listeners are currently not part of the public APIs.

@jonathanolson and I also considered added in isHighlightedProperty to Slider, derived from isHighlightedProperty of the thumb and track DragListeners. But we suspect that there be a need to access other DragListener Properties in the future, so exposing them seems more general and future-proof.

[pixelzoom]

Done in the above commit. @jonathanolson would you mind doing a quick review, since you and I discussed this?

[jonathanolson]

Looks great to me! Thanks.