Improve nuxeo-document-thumbnail

Nuxeo-document-thumbnail was moved from WebUI project to Nuxeo Elements before the Great Element Migration initiative. Because of it, we didn't apply to it the process established for the migration.

Besides this, it is important to rethink the functionality of this element (since it was so tied up to WebUI context). Given this, I suggest to change some of the css proprieties to the host so they can be customised (e.g, border-radius and position).

The goal is to apply the rules described ELEMENTS-1024 and try to move as many styles to :host so that they can be overridden by the parent element.