Enhanced quantize in PianoRoll (#5946)

* Button with menu has split highlight

* Add menu with more action to quantize button

* Style changes

* Fix CSS length-zero-no-unit warning

* Add combo button to classic theme

include/PianoRoll.h

@@ -142,6 +142,13 @@ public:
 	int quantization() const;
 
 protected:
+	enum QuantizeActions
+	{
+		QuantizeBoth,
+		QuantizePos,
+		QuantizeLength
+	};
+
 	void keyPressEvent( QKeyEvent * ke ) override;
 	void keyReleaseEvent( QKeyEvent * ke ) override;
 	void leaveEvent( QEvent * e ) override;
@@ -198,7 +205,7 @@ protected slots:
 	void quantizeChanged();
 	void noteLengthChanged();
 	void keyChanged();
-	void quantizeNotes();
+	void quantizeNotes(QuantizeActions mode = QuantizeBoth);
 
 	void updateSemiToneMarkerMenu();