Adds missing checks to several plugins.

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.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

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

Replaces GLib's GError.

Replaces the old global "soundfont" which never worked.

Now that the libfluidsynth API was sanitized, we can enable the plugin
automatically if libfluidsynth is installed.

Use libfluidsynth's new function fluid_player_get_status().

The libfluidsynth now accepts const strings.

Use fluid_is_midifile() to verify the file format.

The libfluidsynth API is now sane, and does not require real-time
decoding.

Fixes build regression.

Make it X_decoder_plugin.c.