Paint menu scrollers, tear off on top of QWidgetAction items

The menu scrollers and the tear off items are painted in the layer
 of QMenu. In a case that QMenu contains QWidgetAction items, the items
are painted under QWidgetAction items. They are visually hidden unless
QWidgetAction items' background is transparent. The tear off doesn't work
since QWidgetAction item on top of it grabs mouse events. To fix the issue,
add two child widgets in QMenu, paint the scroll up and the tear off items
in one child widget and the scroll down item in the other one. Both child
widgets are painted on top of overlapping sibling menu item widgets.

Change-Id: I7eaef21b667b388fbb78b60664f4a4bd91215309
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>