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.

Eliminate dependency on io_thread_get().

Everything must be S16.

The old struct config_param remains only for top-level string options.

Passing a void pointer is unsafe.

Make it "const".

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

Replaces GLib's GError.