audtag: remove tag_set_verbose() as it's completely disconnected from the codebase
audtag: move audtag public functions to `audtag` namespace.

`tag_update_stream_metadata()` is left in global namespace, instead we mark it with a notice to
investigate moving it to the libaudcore library as it's not related to tagging.
audtag: convert builtin tag modules to C++ objects.

unlike the first attempt, we just do the lame thing and call a setup routine to register everything in the right order.
audtag: give APE builtin priority over id3v1.
Merge pull request #17 from kaniini/master

audtag C++ refactoring v2