Fix nuxeo-dropdown-aggregation with small width

In a results filter, the dropdown list width equals the field width.
This leads to issues when there are many fields in a result table.

The table header styles are defined here:
https://github.com/nuxeo/nuxeo-ui-elements/blob/maintenance-2.2.x/nuxeo-data-table/default-styles.html#L11

They are actually vulkanized into nuxeo-app.html thus preventing any override.

One should be able to set the select width and the option width of a dropdown list.

EDIT Typically reproducible with nuxeo-document-content having all its columsn displayed on a small screen. Click one of the dropdown (i.e. creator, modified, etc.)