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().

Fixes the dreaded error "Failed to create : No such file or
directory".

 https://bugs.musicpd.org/view.php?id=4625

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

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