Use a single attribute for suggestion elements

Since NXS-4655, both attributes selected-items and value are used when generating a suggestion element.
This solution brings a problem when using a multivalued directory: as soon as several values are selected, UI will display ids instead of labels.
There is no possible workaround other than changing the generated configuration, a single value has to be used.

TODO

In Designer, suggestion elements: nuxeo-directory-suggestion, nuxeo-user-suggestion and nuxeo-document-suggestion should use the following property depending on TP

• For new d&d and new layouts, generate code as described above
• For existing, when both values are used, add a validation warning:

  Code generation has changed for suggestion elements. We recommend replacing these elements using drag and drop to benefit from the latest updates and bug fixes.

• Remove existing validation warning when people where using suggestion elements with "value" attribute in 9.10