Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

Fixes build failure on OS X, closes #44.  With the other plugins,
that's not critical, because those use the AudioOutputWrapper, which
hides this problem.

See https://picard.musicbrainz.org/docs/mappings/

This tag is useful when the user would like all releases of the same
album to be sorted next to each other.

Some missing bits when converted to C++ exceptions...

Eliminate dependency on io_thread_get().

Beware, this commit was not tested.  I don't have Haiku, but I want to
prepare an API change.

Avoid one level of dynamic allocation.

Also uses the notification system to display tags.