Clear specific QWindow::focusObject() on QWindowPrivate::clearFocusObject()

The focusObject of a QWidgetWindow is the focusWidget() of the top level
widget, so when clearing the focus object of the window we should clear
focus of the same focusWidget, not the application-wide focus widget,
which may live in another window.

Change-Id: Ib9162418865c225e23aac7987e119b3b651983eb
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>