Add a warning when neither creation nor edition layout use a file widget when document type has the 'file' schema

This probably means that the user forgot it, so it is good to remind him, with an explicit indication like : None of the layout uses the "File" widget: users won't be able to load any file on the document. This should appear only if the document has the file schema, or, even better, if among all the schemas there is a blob property.