fix(files): Remove confusing table header labels

fix(files): Remove confusing table header labels

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>