Fix nuxeo-directory-checkbox behavior after multiple uses

Steps to reproduce

1. In Studio Modeler, define a simple vocabulary with 4 entries, a schema with a multi-valued Directory field and a dcument type using this schema
2. In Studio Designer, configure the View, Edit and Create layouts to use the nuxeo-directory-checkbox elements
3. Deploy the Studio project
4. Go to a Workspace and click on the + button, then choose the new document type
5. In the creation form, select the 2 first checkboxes and submit
6. Observe that the 2 values are correctly displayed
7. Click on the + button and choose the new document type again
8. First problem: the 2 first checkboxes are selected
9. Unselect the checkbox and select the third one and create the doc
10. Second problem: observe that the document was created with 3 values
11. Click on the Edit button, unselect the 3 values and select the 4th one
12. Last problem: observe that the document was saved with the 4 values

Expected behavior: the checkboxes should be correctly reset and only the selected values should be sent to document creation or update.