The possible values of policy are either GTK.UpdateAlways or
GTK.UpdateIfValid.
These policies affect the behavior of a Spin Button when parsing
inserted text and syncing its value with the values of the
Adjustment.
In the case of GTK.UpdateIfValid the Spin Button value only gets
changed if the text input is a numeric value that is within the
range specified by the Adjustment. Otherwise the text is reset
to the current value.
In case of GTK.UpdateAlways errors are ignored while converting text
into a numeric value.