Set up a handler for button or menu item clicks.

        Args:
            handler (func) : handler function to call when button is activated.

        Returns:
            None

def on_click(self, handler):
        ''' Set up a handler for button or menu item clicks.

        Args:
            handler (func) : handler function to call when button is activated.

        Returns:
            None

        '''
        self.on_event(ButtonClick, handler)
        self.on_event(MenuItemClick, handler)