Display OPUS base gain without applying it

[OxygenCobalt]

Kind of niche, but Auxio currently doesn't parse or display an OPUS file's base gain. This used to occur, but it was removed since it wound up conflicting with the system base gain handling, which would also apply the base gain and result in a double application of the ReplayGain adjustment.

Instead, just rejig Auxio's data structures to support the "base gain" without the app actually applying it, rather just displaying it.