replaygain: pass target_level and peak to backends
replaygain: pass target_level and peak to backends

Configure the replaygain analysis by passing arguments to the Backends. This
avoids the difference between ReplayGain and EBU r128 backends; every Backend
can now fulfil both tasks. Additionally it eases Backend development as the
difference between the two tag formats is now completely handled in the main
Plugin, not in the Backends.

sampsyo/beets,ibmibmibm/beets,beetbox/beets,beetbox/beets,shamangeorge/beets,beetbox/beets,shamangeorge/beets,SusannaMaria/beets,sampsyo/beets,jackwilsdon/beets,jackwilsdon/beets,shamangeorge/beets,ibmibmibm/beets,sampsyo/beets,SusannaMaria/beets,SusannaMaria/beets,beetbox/beets,SusannaMaria/beets,ibmibmibm/beets,jackwilsdon/beets,shamangeorge/beets,jackwilsdon/beets,sampsyo/beets,ibmibmibm/beets