QQuickMenu::popup() did not handle the QQuickWidget case properly
when mapping between screen and local coordinates. This meant that
popups appeared in the wrong position for comboboxes, context menus
and edit menus.

Task-number: QTBUG-38116
Change-Id: Iedd7bed55648b1b4a576b9ccb8575b75427bd06f
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>