Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

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.