QQuickText::itemChange must call QQuickItem::itemChange

The need for this was documented, but not done in this case.
d5e612fb3e9753c762b741d135fabd2b1f8ae1a6
So it was not possible to use the windowChanged signal inside a Text.

Change-Id: I4e3a49ca898cbd09b5731bd133cf93c212062fa2
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>