Bug 1029198 - [ringtones] Ringtone Manager should visually indicate whic...

...h ringtone is being previewed
Yeah, I've been thinking about this in light of how we now to use Promises for async. Promises really only make sense for async things that will only happen once, whereas callbacks can make sense even if they're called multiple times.

Maybe it would make more sense if I just added a separate property on the `tonePlayer` called `onplaychange` or something? That would make it look more like the event listener it is.