tag/Handler: convert to class with virtual methods

src/decoder/plugins/FluidsynthDecoderPlugin.cxx

@@ -193,8 +193,7 @@ fluidsynth_file_decode(DecoderClient &client, Path path_fs)
 
 static bool
 fluidsynth_scan_file(Path path_fs,
-		     gcc_unused const TagHandler &handler,
-		     gcc_unused void *handler_ctx) noexcept
+		     gcc_unused TagHandler &handler) noexcept
 {
 	return fluid_is_midifile(path_fs.c_str());
 }