Save configuration: tree widget state

def save_config(self):
        """Save configuration: tree widget state"""
        for option, value in list(self.explorer.get_options().items()):
            self.set_option(option, value)
        self.set_option('expanded_state',
                        self.explorer.treewidget.get_expanded_state())
        self.set_option('scrollbar_position',
                        self.explorer.treewidget.get_scrollbar_position())