Improve UX of document type editor view

Working on CMS-9510, we noticed that it would be better if the tree-view of the document type editor was "rootless", i.e. show no entry for the hippo:namespaces node. This entry currently has the purpose of exposing the "Create namespace" action. When the root node is gone, this action should be triggered by the "+ Add namespace" button at the bottom of the tree view.