Sane add plugin (again)

This will only work in conjunction with https://github.com/divio/django-cms/pull/3516

Changes the way sub plugins are added to text plugins by using a custom view. This view will subclass the plugin class of the plugin to be added so the ``add_response`` method can be overwritten. In that method, we return a response that calls ``CMS.CKEditor.addSubPlugin`` in the parent frame to actually add the plugin.

@FinalAngel would love comments on the JS changes