Fix item-changed listener on nuxeo-data-table not triggering when adding entries from nuxeo-data-table-form

Steps to reproduce:

1. Set up a Studio project with a custom Document type including a multi-valued String schema property. Create and Edit layouts should include a nuxeo-data-table with a nuxeo-data-table-form for adding entries to the property.
2. In Web UI, create/edit an instance of the doctype to view the corresponding layout.
3. In the browser dev tools Sources tab, search for nuxeo-data-table among the source javascript and set a breakpoint at the _itemChanged event listener function.
4. In the create/edit layout, define a new entry to the multi-valued String property and click "OK" to add the entry to the table.

Expected result: breakpoint at _itemChanged is triggered.

Actual result: breakpoint at _itemChanged is not triggered - changes in data table items array are not listened for.