db/simple/Song: add attribute "target"

Will be used for Song objects representing tracks inside a CUE file.

src/db/plugins/simple/DirectorySave.cxx

@@ -161,12 +161,15 @@ directory_load(TextFile &file, Directory &directory)
 			if (directory.FindSong(name) != nullptr)
 				throw FormatRuntimeError("Duplicate song '%s'", name);
 
+			std::string target;
 			auto audio_format = AudioFormat::Undefined();
 			auto detached_song = song_load(file, name,
+						       &target,
 						       &audio_format);
 
 			auto song = std::make_unique<Song>(std::move(detached_song),
 							   directory);
+			song->target = std::move(target);
 			song->audio_format = audio_format;
 
 			directory.AddSong(std::move(song));