Emitted when either:
1. 2 seconds has elapsed since the last value change in the widget
(eg last key press or scroll wheel event)
2. or, immediately after the widget has lost focus after its value
was changed.
This signal can be used to respond semi-instantly to changes in
the spin box, without responding too quickly
while the user in the middle of setting the value.
When a spin box is showing a NULL value, we DON'T do the default step
behavior, as that would add one step to the NULL value, which is usually
a very large negative value... so the user will get a very large negative number!
Instead, treat the initial value as 0 instead, and then perform the step.
