Clickable table headers change sort direction

Make table headers clickable, for changing sorting direction of the entries.
Clicking on the name of the headers selects ascending direction, clicking on
the arrow next to it, chooses descending direction. The arrow symbol, for
simplicity, is an Unicode down-arrow symbol, which all reasonably modern
browsers should display fine. Also, most text-only browsers will show the
arrow correctly when running on Unicode-compatible terminals.