Use new wxAppTraits::SafeMessageBox() in wxSafeShowMessage()

This allows to show message boxes in ports other than wxMSW too by doing
it only when it is safe, i.e. when the GUI is initialized, while still
keeping the old code directly using the native MessageBox() function for
MSW for maximal robustness.

include/wx/msw/apptbase.h

@@ -61,6 +61,10 @@ public:
     // return the main application window or 0 if none
     virtual WXHWND GetMainHWND() const = 0;
 
+    // implement this base class function for both console and GUI applications
+    virtual bool SafeMessageBox(const wxString& text,
+                                const wxString& title) wxOVERRIDE;
+
 protected:
 #if wxUSE_THREADS
     // implementation of WaitForThread() for the console applications which is