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.
(cherry picked from commit 4cad9517aa)

src/core/bb_track_container.cpp

@@ -83,6 +83,11 @@ bool bbTrackContainer::play( midiTime _start, fpp_t _frames,
 
 void bbTrackContainer::updateAfterTrackAdd()
 {
+	if( numOfBBs() == 0 && !engine::getSong()->isLoadingProject() )
+	{
+		engine::getSong()->addBBTrack();
+	}
+
 	// make sure, new track(s) have TCOs for every beat/bassline
 	for( int i = 0; i < qMax<int>( 1, numOfBBs() ); ++i )
 	{