Send QWindow::focusObjectChanged when clearing widget focus

We do this for QWidget::setFocus(), but were missing a call on clearing
the focus widget. Since QWidgetWindow::focusObject() will fall back to
returning itself if there is no focus widget, we need to pass the
result of window->focusObject() to the change signal instead of 0.

Change-Id: I52a5519a19bb20e74b4a7c2a1abc9d47e2ea1315
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>

src/widgets/kernel/qwidget.cpp

@@ -6569,6 +6569,11 @@ void QWidget::clearFocus()
             QAccessible::updateAccessibility(&event);
 #endif
         }
+
+        if (QTLWExtra *extra = window()->d_func()->maybeTopData()) {
+            if (extra->window)
+                emit extra->window->focusObjectChanged(extra->window->focusObject());
+        }
     }
 }