#2516 : add default children for default_plugins of placeholders configured in settings.py.
delete post_add_process and add notify_on_autoadd and notify_on_autoadd_children method to CMSPlugin
clean some mess I let behind me...
Doc correction to be coherent with djangocms-text-ckeditor documentation
Merge pull request #2518 from DylannCordel/issue2516

Add default children for default_plugins of placeholders