Read replay gain tags from more file formats.

We now read replay gain tags from every file format we can read tags from, except for RealMedia (unlikely to have replay gain tags), Audible (does anyone even write replay gain tags to Audible?) and Musepack.

The lack of Musepack support is paricularly ironic, as the reason we can't read replay gain data is that Musepack supports replay gain natively (rather than abusing metadata tags for the purpose), but TagLib won't let us at relevant fields.