ReplayGain support PS

[Tobias Leupold]

... and to make it yet a bit more complicated:

When playing the vorbis file and the "R128_TRACK_GAIN" opus file with Elisa, 
they are both played equally loud (so, no gain adjustment is done).

But when playing the "Playback gain" opus file, it's a lot quieter.

So, as no gain change is implemented yet, it seems that the backend used to 
actually play the file does some volume adjustment for an opus file containing 
a "Playback gain" stream header.

Question is if this can be turned off (if the user doesn't want volume 
adjustment) ... at least, we would have to compensate the different reference 
levels of opus and vorbis; otherwise, even if ReplayGain would be added, opus 
would always be quieter than vorbis ...