Expose nuxeo-data-table column headers with a <th> tag

Column headers in the nuxeo-data-table element are not readable as such by screen readers (e.g., NVDA in Windows).

Expected:

nuxeo-data-table column headers should use a <th> tag (or other appropriate header markup).

It currently uses td elements for all cells. Navigating cell by cell, screen readers will often fail to read the header cells associated with content.

One option to envision is see if we can follow the design pattern guidelines provided by WCAG https://www.w3.org/WAI/ARIA/apg/patterns/table/