editable Tree: make add-item more customizable

* Editing-on-insert might not always be the preferred action
* User might forbid / modify actions in some circumstances
* Async action (show a dialog, IO) might by necessary to create new item

--> add hooks or parameters for on_add_child_cmd
--> could e.g. extend the definition of ``.factory`` - idea: if it returns None, take no further action, assume the user took care of it