Add stream discoverer to gstreamer pipeline and continuous updating of bitrate

src/playlist/playlistmanager.h

@@ -119,6 +119,7 @@ public slots:
 
   // Forwarded from individual playlists
   void CurrentSongChanged(const Song& song);
+  void SongMetadataChanged(const Song& song);
 
   // Signals that one of manager's playlists has changed (new items, new ordering etc.) - the argument shows which.
   void PlaylistChanged(Playlist *playlist);