Add ReplayGain support (volume normalization)

[CreamyCookie]

There was a feature request in 2015 where apparently there were problems with the implementation. Maybe things have improved or there can be a work around. At least it should be possible to read ReplayGain tags and automatically apply a gain change.

[PaulWoitaschek]

The linked thread is about gain. That is implemented.

For replaygain we would need to parse the file headers and constantly adjust the volume:
https://de.wikipedia.org/wiki/Replay_Gain

Up for grabs :P