Add default children for default_plugins of placeholders

#2516

With this diff, we can now add children plugins for default plugins configured in placeholders in settings.py.
Here an exemple of settings.py :

```python
from djangocms_text_ckeditor.utils import plugin_to_tag

def upd_textplugin_ctn_with_child_link(plugin, request, conf):
    textplugin = plugin.parent
    needle = '%%(_tag_%s_%d)s' % (plugin.plugin_type, plugin.position)
    if needle in textplugin.body:
        textplugin.body = textplugin.body.replace(needle, plugin_to_tag(plugin))
        textplugin.save()
            
CMS_PLACEHOLDER_CONF = {
    'main_content': {
        'name': _(u"Contenu principal"),
        'plugins': ['TextPlugin', 'LinkPlugin'],
        'default_plugins':[
            {
                'plugin_type':'TextPlugin', 
                'values':{
                    'body':'<p>Great websites : %(_tag_LinkPlugin_2)s and %(_tag_LinkPlugin_3)s</p>'
                },
                'children':[
                    {
                        'plugin_type':'LinkPlugin',
                        'values':{
                            'name':'django', 
                            'url':'https://www.djangoproject.com/'
                        },
                        'post_add_process':upd_textplugin_ctn_with_child_link,
                    },
                    {
                        'plugin_type':'LinkPlugin',
                        'values':{
                            'name':'django-cms', 
                            'url':'https://www.django-cms.org'
                        },
                        'post_add_process':upd_textplugin_ctn_with_child_link,
                        # If using LinkPlugin from djangocms-link which
                        # accepts children, you could add some grandchildren :
                        # 'children' : [
                        #     ...
                        # ]
                    },
                ]
            },
        ],
    },
}
```

The "post_add_process" is required for some special plugins as textEditor because we have to reference children inside the parent's content. But for a simple plugin (LinkPlugin from djangocms-link), it's not necessary.
An other usefull usage of "post_add_process" is to easy add contextual default content. (with the current user, current page, current lang etc.)

PS : if this pull request is merged, I think it would be usefull to add a function which do the same than "upd_textplugin_ctn_with_child_link" in djangocms-text-ckeditor to easy add children "text-plugin" in textEditor.