Propagate scenery events through slider startDrag and endDrag

[jbphet]

Slider.js takes options that allow clients to specify startDrag and endDrag methods. The scenery input event is not currently passed through to these methods. This is causing an issue in Resistance in a Wire where the isn't able to tell if a slider was moved due to a mouse/touch event versus keyboard interaction (see phetsims/resistance-in-a-wire#204). The slider code should be modified so that the scenery event is passed through to these methods.

[jbphet]

I've added these a couple of months ago and it is working in the specific test case for RIAW, and doesn't seem to have caused any issues elsewhere. This is simple enough that I don't think it requires review.
Closing.