Only use NSCriticalAlertStyle for serious questions

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.

interface/wx/msgdlg.h

@@ -53,7 +53,10 @@ const char wxMessageBoxCaptionStr[] = "Message";
     @style{wxICON_ERROR}
         Displays an error icon in the dialog.
     @style{wxICON_WARNING}
-        Displays a warning icon in the dialog.
+        Displays a warning icon in the dialog. This style should be used for
+        informative warnings or, in combination with @c wxYES_NO or @c wxCANCEL,
+        for questions that have potentially serious consequences (caution
+        icon is used on OS X in this case).
     @style{wxICON_QUESTION}
         Displays a question mark symbol. This icon is automatically used
         with @c wxYES_NO so it's usually unnecessary to specify it explicitly.