Add footer text and icon to wxRichMessageDialog

The underlying Windows TaskDialog supports adding an additional footer
to the message dialog. This makes the native functionality available
and implements it in the generic version.

See https://github.com/wxWidgets/wxWidgets/pull/573

samples/dialogs/dialogs.h

@@ -268,6 +268,8 @@ private:
     wxTextCtrl *m_textCheckBox;
     wxCheckBox *m_initialValueCheckBox;
     wxTextCtrl *m_textDetailed;
+    wxTextCtrl *m_textFooter;
+    wxChoice *m_iconsFooter;
 
     wxDECLARE_EVENT_TABLE();
 };