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/Mapper.hxx

@@ -33,6 +33,7 @@
 class AllocatedPath;
 struct Directory;
 struct Song;
+class DetachedSong;
 
 void
 mapper_init(AllocatedPath &&music_dir, AllocatedPath &&playlist_dir);
@@ -116,6 +117,10 @@ gcc_pure
 AllocatedPath
 map_song_fs(const Song &song);
 
+gcc_pure
+AllocatedPath
+map_song_fs(const DetachedSong &song);
+
 /**
  * Maps a file system path (relative to the music directory or
  * absolute) to a relative path in UTF-8 encoding.