Set numerical input hint on SpinBox

Since we don't allow you to override the text validator, we guarantee
that the only thing you are allowed to type in a SpinBox are numbers.

[ChangeLog][QtQuickControls][SpinBox] SpinBox now only shows the
numerical keys in onscreen keyboards where possible.

Task-number: QTBUG-37619
Change-Id: If6412d1dc49bf0e602330e90963e8cd990f5d43c
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>