DetachedSong: fork of struct Song

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

src/SongSave.hxx

@@ -26,12 +26,16 @@
 
 struct Song;
 struct Directory;
+class DetachedSong;
 class TextFile;
 class Error;
 
 void
 song_save(FILE *fp, const Song &song);
 
+void
+song_save(FILE *fp, const DetachedSong &song);
+
 /**
  * Loads a song from the input file.  Reading stops after the
  * "song_end" line.
@@ -39,8 +43,8 @@ song_save(FILE *fp, const Song &song);
  * @param error location to store the error occurring
  * @return true on success, false on error
  */
-Song *
-song_load(TextFile &file, Directory *parent, const char *uri,
+DetachedSong *
+song_load(TextFile &file, const char *uri,
 	  Error &error);
 
 #endif