Revert secondary sort order when shift-click

[humdinger]

This is hrev52860.

I suspect this is actually a regression, as I remember it working at one time...

• Open a folder, and sort by "Kind".
• Shift-Click on the "Modified" column.

--> Files are sorted by filetype (primary sort), and within the filetypes ascending by date (secondary sort).

• Now shift-click again the "Modified" column.

This should reverse the 2ndary sort order, but instead just removes the triangle overlay in the column header without changing sort order.

Shift-clicking should move through these states:

1. Ascend sorting
2. Descend sorting
3. Remove 2ndary sorting

Also, normal left-clicking any column should remove the secondary sorting. It's a natural way to reset sorting, otherwise you may overlook that you've set a 2ndary sort order. And it's BeOS behaviour, IIRC.