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

interface/wx/richmsgdlg.h

@@ -107,6 +107,54 @@ public:
      */
     wxString GetDetailedText() const;
 
+    /**
+        Shows or hides a footer text that is used at the bottom of
+        the dialog together with an optional icon.
+
+        @param footerText
+            The footer text if empty no footer text will be used.
+
+        @see SetFooterIcon(), GetFooterText()
+        
+        @since 3.1.1
+    */
+    void SetFooterText(const wxString& footerText);
+
+    /**
+        Retrieves the footer text.
+
+        @return
+            The footer text or empty if footer text is not used.
+
+        @since 3.1.1
+    */
+    wxString GetFooterText() const;
+
+    /**
+        Specify the footer icon set together with the footer text.
+
+        Valid values are @c wxICON_INFORMATION, @c wxICON_WARNING,
+        @c wxICON_AUTH_NEEDED and @c wxICON_ERROR (notice that
+        @c wxICON_QUESTION is not allowed here).
+
+        @see GetFooterIcon(), SetFooterText()
+
+        @since 3.1.1
+    */
+    void SetFooterIcon(int icon);
+
+    /**
+        Retrieves the footer icon.
+
+        @return
+            The footer icon or 0 if footer icon is not used.
+
+        @see SetFooterIcon()
+
+        @since 3.1.1
+    */
+    int GetFooterIcon() const;
+
     /**
         Retrieves the state of the checkbox.