replaygain: Convert the ad-hoc tag_specific_values to classes
replaygain: Convert the ad-hoc tag_specific_values to classes

The plugin has loads of indirection and nested functions which make it
really hard to reason about. The larger picture here is that I'd like to make
the code more manageable before reworking the parallelism issues.

In particular, instead of manually implementing an interface using a
function that returns a tuple of function pointers, this commit creates proper
classes. Again, no functionality is changed, this only moves code
around.

beetbox/beets,beetbox/beets,beetbox/beets,beetbox/beets