dataengines/mpris2: Snap volume to step increments

As per plasma-pa!158 (closed), this commit makes the media controller applet respect volume step increments. This is done in the data engine's changeVolume function rather than in the QML code that calls it.

I'm not sure if this is the appropriate place to do this (it might be done in the applet), nor if the code is particularly readable - but it does work.