Fix Spinbox missing value change if minimumValue is set

The Spinbox validator considered invalid an intermediate value preventing the user for inserting a valid value.

Task-number: QTBUG-42342
Change-Id: I24d2f9f7d6fb9d5d4f96f81821b37a30421665a3
Reviewed-by: Filippo Cucchetto <filippocucchetto@gmail.com>
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>