[audio] Add settings option to boost centre channel when downmixing

This allows a dB volume increase to be added to centre channel.
This can help improve dialgue in the presence of background music/effects.
It can go up to 30dB for testing purposes, but value of 6 is probably more reasonable.
It is recommended to ensure "Normalise levels on downmix" is enabled when boosting by large values to avoid clipping.

Should work with Pi Sink (dvdplayer/paplayer) and omxplayer