Fixes valgrind warning.

Fixes another part of #208, the one which caused the bogus exception.

Wired to Filter::Flush().

Closes #153

These are not in filter_plugins[] and have dedicated constructors
anyway.

Commit 3a212412 changed the meaning of out_audio_format, but I forgot
to apply this to ConvertFilter::FilterPCM().

See https://bugs.musicpd.org/view.php?id=4551

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.

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

Replaces GLib's GError.