Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

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

Check for STOP before decoding the first chunk.  This reduces the
command latency.

Remember the chunk's total size.

Nobody cares.

Eliminate the loop from dsdiff_stream_decode().  It makes the code
complex, real-world files with multiple DSD chunks are outside of the
specification, and the "chunk_size" variable would be bogus anyway.

Reduce header dependencies.

They are only used inside dsdiff_read_metadata_extra().

Merge lots of duplicate code.

Decouple some more from GLib.