For screen readers the table header was very verbose and confusing,
as the SR reads out e.g. "sort list by name button FILENAME" for every row / file.
Instead reduce it to "name button FILENAME" and add information about sorting to caption, as recommended by WCAG [1]
[1]: https://www.w3.org/WAI/ARIA/apg/patterns/table/examples/sortable-table/
Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>