QQuickWindow should not clear its focusObject when the window looses focus

QWindow::focusObject() represents the object that should get focus when the window becomes active(focused). QWidgetWindow follows this pattern and still maintains a valid focusObject even on a FocusOut event. QQuickWindow OTOH, closely ties the focusObject to the activeFocusItem so whenever the window looses focus the focusObject becomes invalid. We should change it to have a consistent behaviour across widgets and Quick.