Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

The `tag_builder.empty()` check was wrong for the SubmitReplayGain()
call.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/497

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.

Prepare to reuse the same code for a new Vorbis decoder plugin based
on libvorbis instead of libvorbisfile.

Initialize the OggStreamState as soon as the first page is seen; no
special code needed in the caller.