Fix log ERROR when using the Files widget

Context

When using the Files widget from the files schema as described by NXP-25810, we get these errors in the logs:

2018-11-21T14:56:05,906 ERROR [RemovedProperty] Property 'files/-1/filename' is marked as removed from 'files' schema, don't use it anymore. Return null
2018-11-21T14:56:05,906 ERROR [RemovedProperty] Property 'files/-1/filename' is marked as removed from 'files' schema, don't use it anymore. Do nothing

The fix is probably to use file/name instead of the old filename property when generating the files widget:

      <widget name="files" type="filesList">
        <labels>
          <label mode="any">Files</label>
        </labels>
        <translated>false</translated>
        <fields>
          <field>files:files</field>
        </fields>
        <subWidgets>
          <widget name="sub0" type="file">
            <labels>
              <label mode="any"></label>
            </labels>
            <translated>false</translated>
            <fields>
              <field>file</field>
              <field>file/name</field>
            </fields>
          </widget>
        </subWidgets>
      </widget>

Reproduction

Steps for reproduction:

1. Download Nuxeo Server 9.10
2. Install JSF UI
3. In Studio, create a custom Document Type called "JsonFiles", and make sure it extends the schema "files"
   1. Once the document type created, navigate to tab "Schema"
   2. Add extra schemas to this document type: "files"
   3. Go to "Creation Layout"
   4. Replace "WARNING" by "Files" widget. "Files" widget can be selected on the right panel: Widget by Property > Schemas > files.
   5. Click button "Save work in progress"
   6. Do the same thing for "Edit Layout"
   7. Do the same thing for "View Layout"
4. Sync the project with the server
5. Create a document and attach 2 JSON files using the button "Add New Attached File"
6. Save the document
7. Observe the server logs contain:

   2019-04-25 17:20:38,907 ERROR [RemovedProperty] Property 'files/-1/filename' is marked as removed from 'files' schema, don't use it anymore. Return null
   2019-04-25 17:20:38,907 ERROR [RemovedProperty] Property 'files/-1/filename' is marked as removed from 'files' schema, don't use it anymore. Do nothing
   2019-04-25 17:20:38,907 ERROR [RemovedProperty] Property 'files/-1/filename' is marked as removed from 'files' schema, don't use it anymore. Return null
   2019-04-25 17:20:38,907 ERROR [RemovedProperty] Property 'files/-1/filename' is marked as removed from 'files' schema, don't use it anymore. Do nothing