Every widget now has a GUI::FocusPolicy that determines how it can
receive focus:
- NoFocus: The widget is not focusable (default)
- TabFocus: The widget can be focused using the tab key.
- ClickFocus: The widget can be focused by clicking on it.
- StrongFocus: Both of the above.
For widgets that have a focus proxy, getting/setting the focus policy
will affect the proxy instead.
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
