Fix nuxeo-directory-checkbox not being cleared by reset button from nuxeo-search-form

Steps to Reproduce:

1. In Studio, set up a Page Provider with a predicate on a directory attribute (e.g. dc:nature, dc:subjects).
2. In the search form layout for the Page Provider, use a nuxeo-directory-checkbox element to take input for the field.
3. Bind the search form to a new Drawer item.
4. In Web UI, navigate to the Drawer item and mark 1+ checkboxes.
5. Click the Reset button at the bottom of the nuxeo-search-form pane.

Expected Results: the checkboxes of nuxeo-directory-checkbox are cleared

Actual results: the checkboxes of nuxeo-directory-checkbox are not cleared