Thsi filter doesn't make sense as manually configured plugin, because
nobody ever calls volume_filter_set() on it, making it a no-op.

Circumvent filter_new() with a fake ConfigBlock.

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.

Prepare for adding state.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Eliminate support for reverse endian samples from the MPD core.  This
moves a lot of complexity to the plugins that really need it (only
ALSA and CDIO currently).

The other audio_format attributes are not used.