By default, a SpinBox's value should be unlimited, (or as close as we
can get to that,) and then the GML or code can impose a limit if
needed. This saves the developer from entering an arbitrary "big" max
value when they want the value to have no maximum.
I've audited the use of SpinBox and added `min: 0`, and removed a `max`,
where appropriate. All existing SpinBoxes constructed in code have a
range set explicitly as far as I can tell.
Since the meridiem selection was removed in the previous commit and
there currently doesn't seem to be a way to change the system time
format, I'm changing the SpinBoxes here to accept 24-hour format values
for now.
