Fix incorrect spinbox validation at startup

The problem was that the validator was applied during the
object creation which means the incorrect decimal values were applied
and the initial value was incorrectly changed.

Task-number: QTBUG-33309
Change-Id: Ic1ace174b9059b9c6ce24fd88b81c5edd7318e80
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>