For easier state management inside filter plugins.

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

API simplification.  We can now avoid abusing a "size_t*" as
additional return value.

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

This prevented using the "volume_normalization" feature with some
codecs (e.g. mp3), because the normalization code requires 16 bit
samples.  If the codec happens to deliver formats other than S16, the
AutoConvert filter succeeds to initialize the conversion filter, but
the returned input audio format was wrong.

Replaces GLib's GError.