add notify_on_autoadd_children for issue divio/django-cms#2516

Related to django-cms pull request : divio/django-cms#2518

Implement the notify_on_autoadd_children method to be able to easy add children plugins to ckeditor via python code. The body must contain as many %(_tag_child_<order>)s as you have children to add. "<order>" is the place of the plugins when you add it. ex :

```python
textPlugin.body = 'test %(_tag_child_1)s and %(_tag_child_2)s
aChild.parent = textPlugin
anotherChild.parent = textPlugin
orderedChildren = list(anotherChild, aChild)
#tag content of anotherChild will be referenced by %(_tag_child_1)s and aChild by %(_tag_child_2)s
testplugin.notify_on_autoadd_children(request, conf, orderedChildren)
```