This behaviour might be not completely intuitive, but it makes it much
simpler to handle the box state using a checkbox as the label control
(which is by far the most common case of using box window labels).
Notice that while we could add a separate EnableWithoutLabel() method to
wxStaticBox to make it possible to set the state of the box directly
relatively easily, it wouldn't help with using wxEVT_UPDATE_UI for
managing the box state indirectly as it relies on calling Enable() only.
And this solution does allow wxEVT_UPDATE_UI handlers for the box itself
to work (provided the handler takes care to check for the event object
being the box itself, as otherwise it would still disable the child
checkbox when its wxEVT_UPDATE_UI bubbles up to the box).
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