add a listener to the widget
            Args:
                listener_type: string that can either be 'objectHovered' or 'objClicked' 
                callback: python function

def add_listener(self, listener_type, callback):
        """ add a listener to the widget
            Args:
                listener_type: string that can either be 'objectHovered' or 'objClicked' 
                callback: python function"""
        self.listener_type= listener_type
        if listener_type == 'objectHovered':
            self.listener_callback_source_hover= callback
        elif listener_type == 'objectClicked':
            self.listener_callback_source_click= callback
        elif listener_type == 'click':
            self.listener_callback_click= callback
        elif listener_type == 'select':
            self.listener_callback_select= callback

        self.listener_flag= not self.listener_flag