Allow sorting by clicking column headers

[bmcdonnell-fb]

Feature request as in title.

• First click on a header - sorts by that column, ascending order
• Subsequent consecutive clicks on the same column header - reverse sort order
• Optional: multi-column sort (i.e. sort primarily by column X; if X1==X2, then sort by column Y)
  • Possible implementations:
    • user clicks column headers from least to most significant
    • right click header → sort level 2/3/etc