Enhanced FormsManager (cascade process) and Editors.

Enhanced FormsManager:
- added `cascadeEditor()` method to ease the cascading of a document (existing or new)
- added `cascadeEdit(doc)` and `cascadeCreate(modelName or doc)` in Editors' scope (directly available in the template).

Enhanced Editors: added possibility to **hide a section in the MainMenu when a document is being created** (add `hide-when-create="true"` on the `<fieldset/>` in the Editor template).