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/apptrait.h

@@ -88,6 +88,16 @@ public:
     // return true to suppress subsequent asserts, false to continue as before
     virtual bool ShowAssertDialog(const wxString& msg) = 0;
 
+    // show the message safely to the user, i.e. show it in a message box if
+    // possible (even in a console application!) or return false if we can't do
+    // it (e.g. GUI is not initialized at all)
+    //
+    // note that this function can be called even when wxApp doesn't exist, as
+    // it's supposed to be always safe to call -- hence the name
+    //
+    // return true if the message box was shown, false if nothing was done
+    virtual bool SafeMessageBox(const wxString& text, const wxString& title) = 0;
+
     // return true if fprintf(stderr) goes somewhere, false otherwise
     virtual bool HasStderr() = 0;
 
@@ -208,6 +218,8 @@ public:
 
     virtual bool ShowAssertDialog(const wxString& msg) wxOVERRIDE;
     virtual bool HasStderr() wxOVERRIDE;
+    virtual bool SafeMessageBox(const wxString& text,
+                                const wxString& title) wxOVERRIDE;
 
     // the GetToolkitVersion for console application is always the same
     wxPortId GetToolkitVersion(int *verMaj = NULL,
@@ -248,6 +260,13 @@ public:
     virtual bool ShowAssertDialog(const wxString& msg) wxOVERRIDE;
     virtual bool HasStderr() wxOVERRIDE;
 
+    // Win32 has its own implementation using native message box directly in
+    // the base class, don't override it.
+#ifndef __WIN32__
+    virtual bool SafeMessageBox(const wxString& text,
+                                const wxString& title) wxOVERRIDE;
+#endif // !__WIN32__
+
     virtual bool IsUsingUniversalWidgets() const wxOVERRIDE
     {
     #ifdef __WXUNIVERSAL__