Fix view layout generation for Folderish document types

Steps to reproduce:
Modeler:
1. Create a new document type.
2. Set "Document is folderish" under "Document Facets".
3. In "Accepted Children Types" add "File".
Designer:
4. Configure all layouts for this document type (automatically).
Deploy the configuration to a 10.10 server (with Web UI):
5. Create a new document of the newly created type.
6. Add a couple of "File" documents inside.
7. Notice how the children files don't appear anywhere. Inspecting the code of the view layout for this document type, only one nuxeo-page-provider element is specified. No nuxeo-results to display the children.