SongEditor: remove unused messing with engine's pointer to SongEditor

Back in time we obviously didn't manage to get construction dependencies
sorted so we needed to mess with the pointer to the SongEditor instance in
the engine class.

Thanks to Alexandre Almeida for pointing out this issue.

Closes #261.

include/song_editor.h

@@ -56,7 +56,7 @@ class songEditor : public TrackContainerView
 {
 	Q_OBJECT
 public:
-	songEditor( song * _song, songEditor * & _engine_ptr );
+	songEditor( song * _song );
 	virtual ~songEditor();