to be used eventually by class of object adopting an html_void object
- html_void is a transparent type of object (it do not generate any HTTP body outside what its adopted children produce). It is used to provide a global visibility switch to objects it has adopted
- html_void_parent_notifier is an interface. An object adopting an html_void component can implement (but is not obliged to) this interface to be notified of status changes of the html_void object, by implementing (overriding) the specified methods of that class. if parent object of an html_void implements this interface it will be notified of the events this html_void hits. Only the object that has adopted a given html_void is notified of events met that html_void.