Fix missing readonly mode on nuxeo-file when generating layouts for multi property

Steps to reproduce:

• Define a document type with a multivalued complex field with 2 subfields:
  file: Blob
  name: String
• Go to designer
• Generate the create layout for this new doc type
• See generated layout for the complex multivalued field:

      <nuxeo-data-table role="widget" label="Complex" items="{{document.properties.test:complex}}" orderable="true" editable="true">
        <nuxeo-data-table-column name="String">
          <template>
            [[item.string]]
          </template>
        </nuxeo-data-table-column>
  
        <nuxeo-data-table-column name="Blob">
          <template>
            <nuxeo-file role="widget" value="[[item.blob]]"></nuxeo-file>
          </template>
        </nuxeo-data-table-column>
  
        <nuxeo-data-table-form>
          <template>
            <nuxeo-input value="{{item.string}}" label="String" type="text"></nuxeo-input>
            <nuxeo-file value="{{item.blob}}"></nuxeo-file>
          </template>
        </nuxeo-data-table-form>
      </nuxeo-data-table>
  

  KO.

In

      <nuxeo-data-table-column name="Blob">
        <template>
          <nuxeo-file role="widget" value="[[item.blob]]"></nuxeo-file>
        </template>
      </nuxeo-data-table-column>

the nuxeo-file widget should have the readonly attribute. Indeed the user is supposed to open the table form/dialog to edit the blob. Side effects is people think they can do inline editing for multi complex props but it is not (even never been) supported by our nuxeo-data-table.