Add BB track when adding track to empty BB editor

When there's no BB track but a track is added to the BB editor,
automatically create a BB track so that the user doesn't get confused
by new empty track (not even showing clickable beats).

Partially closes #3149779.

include/song.h

@@ -215,11 +215,12 @@ public slots:
 
 	void clearProject();
 
+	void addBBTrack();
+
 
 private slots:
 	void insertBar();
 	void removeBar();
-	void addBBTrack();
 	void addSampleTrack();
 	void addAutomationTrack();