Fix for Icons and comboboxes mismatch in arpeggiator in Instrument Editor #5494 (#5623)

* Fix for Icons and comboboxes mismatch in arpeggiator in Instrument Editor #5494
(https://github.com/LMMS/lmms/issues/5494)

Introduce a static const int variable for the default height of a ComboBox.
Set this height already in the constructor of the ComboBox object.
Update all modules setting the height of a ComboBox object to make use of the new constant.

* Replace 'const int' by 'constexpr int' after review.

include/ComboBox.h

@@ -32,8 +32,6 @@
 #include "ComboBoxModel.h"
 #include "AutomatableModelView.h"
 
-
-
 class LMMS_EXPORT ComboBox : public QWidget, public IntModelView
 {
 	Q_OBJECT
@@ -51,6 +49,8 @@ public:
 		return castModel<ComboBoxModel>();
 	}
 
+	static constexpr int DEFAULT_HEIGHT = 22;
+
 public slots:
 	void selectNext();
 	void selectPrevious();