Convert the fill_buffer to a class method, to allow accessing the private method.

Also remove the leftover Wrapper stuff.

By API contract, this parameter must not be zero.

Adds missing checks to several plugins.

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.