Don't use NSCriticalAlertStyle for wxICON_WARNING unconditionally, this
was a violation of the OS X guidelines. According to the hig HIG,
NSCriticalAlertStyle (aka caution icon) is only appropriate in rare
cases and only if the user is performing a task that might result in the
inadvertent and unexpected destruction of data.
It therefore doesn't make sense to use it for information warnings, but
only for _questions_, and so NSCriticalAlertStyle is only used if a
combination of wxICON_WARNING with either wxYES_NO or wxCANCEL is used.
6b8b3ee379