Enable view-based customization of nuxeo-data-grid

Customizing the cells/thumbnails of nuxeo-data-grid currently requires:

1. creating a customized nuxeo-document-grid-thumbnail
2. including the custom element in the custom bundle .html
3. editing the nuxeo-data-grid instance of the view layout to include the custom element

each of which requires the code view to accomplish - contrasting with nuxeo-data-table, which allows visual customization through the table editor.

Requesting similar visual customization capabilities for nuxeo-data-grid.