Fix inconsistent leave/enter events for main window when popup is open

Fix inconsistent leave/enter events for main window when popup is open

Changed the platform leave event handling logic in QWidgetWindow to
match platform leave event handling logic in Qt4, where last mouse
receiver is used as leave target only if last mouse receiver wasn't
a native window itself. In that case it is assumed to get leave event
of its own when relevant.

Task-number: QTBUG-27639
Change-Id: Id6edcd29754a15c959f18ab38b20d66e5d446510
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>