Fixes keyboard increment/decrement of SpinBox values.
After PR #2412 the TextBox class started not propagating arrow key
events to the parent widgets because it handles them itself now.
It also added two new events for these arrow keys, so use them instead
in SpinBox.
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
