Specifies the name of the action with which this widget should be associated.
action_name is null then the widget will be unassociated from any previous action.
Usually this function is used when the widget is located (or will be located) within the hierarchy of a ApplicationWindow.
Names are of the form “win.save” or “app.quit” for actions on the containing ApplicationWindow or its associated Application, respectively. This is the same form used for actions in the Menu associated with the window.
a Actionable widget
an action name, or null