Improve Fusion style ComboBox UX: Popup Positioning

The popup should either be positionend under or above the control to give a clear distinction between the currently selected item and the options in the list and to avoid accidental selection.

macOS does this best: It positions the combobox popup so that the currently selected item is located directly over the control. If screen constraints don't allow such a positioning, this is somewhat relaxed but still works ok.

Windows always positions the popup above or below the control, whatever screen constraints permit. Thus the current value is always displayed twice (control + popup) but it works ok nevertheless.

Both solutions in any case ensure that a second click at the same cursor position never changes the selected item (macOS: If not possible otherwise the popup is positioned above the mouse cursor position so a second click is surely outside the popup).

With Fusion it's very easy to accidentally change the current item when the user only wanted to peek into the list of available options.

QTBUG-68794 fixed this partially by making a better distinction between hovered and selected items but it does not fix the general usability problem.