Enable UI sound, review the result

[jbphet]

The design document for this sim says, "Start with supportsSound=true and evaluate. If sim sounds empty or provides insufficient feedback, will either progress to full sound design or turn off sounds entirely." @AgustinVallejo and I will now turn on sound and start this process.

[jbphet]

Sound has now been turned out and can be tried out on phettest. I took it for a test drive and IMO it's okay, but not great. Probably better than nothing. I have a few thoughts about how to improve things without a full sound design:

• It would be nice to use a non-default sound for the green arrow button and the "New Coin" button on the 1st screen. Having all the buttons, including those on the tweakers, use the same sound seems repetitive. We could probably just use some existing sound for this.
• The momentary buttons for firing the photons and the individual particles on screens 2 & 3 makes two sounds, one when pressed and one when released. It's a bit much. It would be nice to turn off the release sound.
• The sliders on the first and second screen would benefit from having a few more ticks. The default number seems a bit low.
• It would be nice to have a "photon detected" sound for the "Single Photon" mode in screen 2.
• It might be good to have a low, non-intrusive sound for photons reaching the detector in the "Many Photons" mode of screen 2.

Assigning to @arouinfar for review and discussion of next steps, including whether to do the items listed above.