Add more context in nuxeo-document-layout

Currently, we only pass to nuxeo-document-layout.html a shell document containing the type to be created:

 <nuxeo-document-layout id="document-create" layout="create" document="[[document]]"></nuxeo-document-layout>

(https://github.com/nuxeo/nuxeo-web-ui/blob/master/elements/document/nuxeo-document-create.html#L200)

We must give more context for instance:

• currentDocument (or at least the target path). It can be needed to retrieve information about the parent document that are necessary to fill/prefill some field in the creation form.