LightSong: add attribute "real_uri"

The UPnP database plugin can now show relative song URIs for remote
songs.

src/Mapper.cxx

@@ -223,7 +223,7 @@ map_song_detach(const LightSong &song)
 {
 	DetachedSong detached(song);
 
-	if (detached.IsInDatabase()) {
+	if (detached.IsInDatabase() && !detached.HasRealURI()) {
 		const auto uri = song.GetURI();
 		detached.SetRealURI(PathTraitsUTF8::Build(music_dir_utf8.c_str(),
 							  uri.c_str()));