Translate "Message" as default title of message box...

... This required a sweeping change of all calls to wxMessageBox!  But it seems
safe to me, despite the great number of touched files.

src/widgets/ErrorDialog.cpp

@@ -232,3 +232,8 @@ void ShowAliasMissingDialog(AudacityProject *parent,
    // PRL: answer is that the parent window guarantees destruction of the dialog
    // but in practice Destroy() in OnOK does that
 }
+
+extern wxString AudacityMessageBoxCaptionStr()
+{
+   return _("Message");
+}