@returns Returns wxCHK_UNCHECKED when the checkbox is unchecked,
wxCHK_CHECKED when it is checked and
wxCHK_UNDETERMINED when it's in the undetermined state.
@returns Returns wxCHK_UNCHECKED when the checkbox is unchecked,
wxCHK_CHECKED when it is checked and
wxCHK_UNDETERMINED when it's in the undetermined state.