axAreaHelper: provide an unregister method

[jpommerening]

The unregister() method would be used by an adapter to unregister disappearing widget areas before they are destroyed. For example, if they are nested in a v-if/ng-if.
The area helper could then move the widgets from the area into an off-screen element, maybe triggering visibility events before the technology adapter destroys the DOM node associated with the area.
Once the adapter decides to register another element with the same name the area helper could just add the hidden elements to the area. These hidden elements need to be released as soon as the "owner" of the axAreaHelper is destroyed (either the page, or the widget(-adapter) that received the axAreaHelper as an injection).