QQuickPopupWindow: Use actual item size as content size

Using the children rect size was a mistake, particularly when using
Instantator in a Menu.

Task-number: QTBUG-40221
Change-Id: I6efe9aad617a7b0adbea0bb96bf843c5c1779131
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>