playlist: don't add entries in reverse

The entries were always added after the insertion point - but that
means the entries are appended in reverse order. So update the
insertion point on each entry.

Regression introduced by commit 5f664d7.

core/playlist.c

@@ -208,6 +208,7 @@ void playlist_transfer_entries(struct playlist *pl, struct playlist *source_pl)
         struct playlist_entry *e = source_pl->first;
         playlist_unlink(source_pl, e);
         playlist_insert(pl, add_after, e);
+        add_after = e;
     }
 }