Don't emit onValueChanged during construction of SpinBox.

Task-number: QTBUG-33504

Change-Id: I9860be6524ec82274f7d256d65aaef3f7657a09b
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>