Allows to define an icon for the App
        
            Args:
                base64_data (str): base64 encoded image data  (ie. "data:image/x-icon;base64,AAABAAEAEBA....")
                mimetype (str): mimetype of the image ("image/png" or "image/x-icon"...)
                rel (str): leave it unchanged (standard "icon")

def set_icon_data(self, base64_data, mimetype="image/png", rel="icon"):
        """ Allows to define an icon for the App
        
            Args:
                base64_data (str): base64 encoded image data  (ie. "data:image/x-icon;base64,AAABAAEAEBA....")
                mimetype (str): mimetype of the image ("image/png" or "image/x-icon"...)
                rel (str): leave it unchanged (standard "icon")
        """
        self.add_child("favicon", '<link rel="%s" href="%s" type="%s" />'%(rel, base64_data, mimetype))