Move and refactor code from the Wavpack decoder plugin.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Move code from the "mad" plugin.

Move code from the "mad" plugin.

Replaces Reset() and eliminates the AudioFormat parameters from the
Convert() method.

This commit adds the basic infrastructure for reporting bugs from
DecoderAPI.cxx via DecoderThread.cxx to DecoderControl.

Reduce header dependencies and allow it to be nullptr to disable it.

Merge duplicate code.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Avoid duplicating the tag.