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.

This function replaces the replay_gain_info parameter for
decoder_data().  This allows the decoder to announce replay gain
changes, instead of having to pass the same object over and over.

Remove the data_time parameter from decoder_data().  This patch
eliminates the timestamp counting in most decoder plugins, because the
MPD core will do it automatically by default.

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.