I'm not sure why developers continue to believe that "tree of widgets!" is a good design for a CMS. It's horrendous.
First, most CMS users are end-clients who will have absolutely no clue to handle a drag-and-drop dependent UI, let alone a nested tree of UIs.
Second, it encourages a site structure where the content is stored hierarchically according to display requirements, instead of the actual semantic relationship between elements.
While I'm sure this is very well coded, it continues the long tradition of content management systems built by developers for developers and in such a way only other developers will like or understand.