Mapper: add function map_song_detach()

Make the DetachedSong(Song) conversion constructor private.  Everybody
should use map_song_detach() which will take over more
responsibilities soon.

src/Mapper.hxx

@@ -106,6 +106,14 @@ gcc_pure
 AllocatedPath
 map_directory_child_fs(const Directory &directory, const char *name);
 
+/**
+ * "Detach" the #Song object, i.e. convert it to a #DetachedSong
+ * instance.
+ */
+gcc_pure
+DetachedSong
+map_song_detach(const Song &song);
+
 /**
  * Determines the file system path of a song.  This must not be a
  * remote song.