Reinstate Double Click opening the piano roll from song editor

This fixes a bug introduced when removing double click from
BB patterns. It Now checks if we use fixedTCO's (bb tomb stones),
only disable double clikc if so, leaving it working correcly in the
song editor

src/tracks/Pattern.cpp

@@ -846,6 +846,19 @@ void PatternView::mousePressEvent( QMouseEvent * _me )
 	}
 }
 
+void PatternView::mouseDoubleClickEvent(QMouseEvent *_me)
+{
+	if( _me->button() != Qt::LeftButton )
+	{
+		_me->ignore();
+		return;
+	}
+	if( !fixedTCOs() )
+	{
+		openInPianoRoll();
+	}
+}
+