QQuickFlickable: Send synthetic release event through QCoreApplication

As we do this with the press event, we have to do the same with the
release event. The application tracks the state of mouse buttons and
will get confused if an additional press event without symmetric
release event shows up.

Change-Id: Ia27a96fa5b207ed8aaad913caf7575de112e8606
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

src/quick/items/qquickflickable.cpp

@@ -1251,7 +1251,7 @@ void QQuickFlickable::mouseReleaseEvent(QMouseEvent *event)
             if (window() && window()->mouseGrabberItem()) {
                 QPointF localPos = window()->mouseGrabberItem()->mapFromScene(event->windowPos());
                 QScopedPointer<QMouseEvent> mouseEvent(QQuickWindowPrivate::cloneMouseEvent(event, &localPos));
-                window()->sendEvent(window()->mouseGrabberItem(), mouseEvent.data());
+                QCoreApplication::sendEvent(window(), mouseEvent.data());
             }
 
             // And the event has been consumed