R-O QComboBox popup could look better on macOS

There are problems with the checkmark. It should be 2px higher, and the spacing between the checkmark and the text 2px wider. Also, the spacing from the left edge of the popup and the checkmark should be 2-3px wider. The checkmark pixmap looks not 100% right, but I suspect it's a Carbon problem. The rest looks OK, but maybe the popup should be wider as well.