Let users sort across multiple columns

====
imported from sf tracker
id 830363
submitted by Richard Atkins - rjatkins
http://sourceforge.net/support/tracker.php?aid=830363
====

 Sorting in the library should support the user sorting
across multiple columns. A sort from one display of a
table should be used as input for the sort on the next
column.

eg. From the initial table (warning: ugly ascii art follows)
Name | Dept | Role
=======================
Bob | Sales | Manager
Joe | Dev | Engineer
Fred | Dev | Manager

Sorting this by Name gives:
Name | Dept | Role
=======================
Bob | Sales | Manager
Fred | Dev | Manager
Joe | Dev | Engineer

Sorting this by Dept gives:
Name | Dept | Role
=======================
Fred | Dev | Manager
Joe | Dev | Engineer
Bob | Sales | Manager

Note that Fred now comes *before* Joe in the list.

Implementing this would mean that sorting would work in
the same way that it does in standard windows list
views, so would work the way many users would expect.