Fix complex list property editing with required field

Steps to reproduce:

• create a complex list property in a doc type
• create a layout to edit this complex list property and make on the field required and using the nuxeo-data-table with editable and orderable. But instead of relying on popup editing, do some inline editing by using a paper-input in the nuxeo-data-table-column template.
• in the Web UI, go to edit this metadata
• add a new Entry/Row
• remove it
• click save
• see that it does not do anything

The nuxeo-data-table uses an iron-list to display the items and to make it performant, it reuses the DOM nodes to display items. As a matter of fact, when you remove an item, the associated row stays in the tree and the validation is performed on it.

There's a silent validation error which prevents the form from being saved.

Alternative is to implement a home made list editing based on dom-repeat templae