ReplayGain support

[jansol]

Counts as a must-have feature for me. With the wide variety of styles in my library a plain volume setting isn't sufficient as files can have very different loudness (especially in shuffle all mode which I use a lot).

The usual "auto" behavior for replaygain is to use track gain when shuffle is on and album gain when it's not. Most players also allow users to choose auto, always track gain and always album gain.

The easiest way to handle this is probably to modulate the user-set playback volume with the track/album gain found in the file's tags. I believe youtube is doing something like this, considering that they show the set/adjusted volume in Stats for Nerds.