Specify target volume for ReplayGained playback & permanent volume adjustment

[Guillaume]

As specified here, I'd love to see the following with regard to ReplayGain handling:

1) Incorporate a target volume dB adjustment setting for playback in MB (default is 89dB, which is very low)
2) Incorporate a target volume dB adjustment setting for permanent volume adjustment, which enables permanent volume adjustment to e.g. 6dB above normale with this parameter set to 95dB.