Consider the following: upon instanciation of a GUI::SpinBox, its
internal value and displayed value are both 0. When calling `set_min(1)`
on it (same as `set_range(1, max())`), the internal value gets clamped
to 1 correctly, but "0" is still displayed by the widget.
The displayed value is now updated accordingly to the internal value
when it gets clamped.
Get rid of the weird old signature:
- int StringType::to_int(bool& ok) const
And replace it with sensible new signature:
- Optional<int> StringType::to_int() const
