Header accessibility issues

[brunnerh]

Describe the issue

• The header elements cannot be focused via the keyboard through Tab. Sortable headers should be focusable and behave as if the user clicked on them upon pressing Enter.
• The filter buttons in the header are focusable but their visibility is not tied to focus. They are only visible upon mouse hover.

To Reproduce
[Minimal example]
I added a style to add a red border on the header cells when the focus is on a filter button within the cell.

• Tab around, the header itself will never receive focus.
• Tab until a filter button receives focus. It will only become visible on mouse hover.

[revolist]

Can be done as part of WCAG plugin for Pro version.