Fix MIDI export (#3733)

* Re-enable MIDI export

* Fix logic for processing BB tracks and BB notes

* Consider master pitch and base note in MIDI export.

* Cut BB notes at the end of BB pattern.

include/ExportFilter.h

@@ -39,7 +39,9 @@ public:
 	virtual ~ExportFilter() {}
 
 
-	virtual bool tryExport( const TrackContainer::TrackList &tracks, int tempo, const QString &filename ) = 0;
+	virtual bool tryExport(const TrackContainer::TrackList &tracks,
+				const TrackContainer::TrackList &tracksBB,
+				int tempo, int masterPitch, const QString &filename ) = 0;
 protected:
 
 	virtual void saveSettings( QDomDocument &, QDomElement & )