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.

For type safety and code readability.

Move to fixed-point integers instead of floating point.

Move and refactor code from the Wavpack decoder plugin.

Move code from the "mad" plugin.

Move code from the "mad" plugin.

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

Avoid duplicating the tag.