Memorize current sorted column when sorting by click

[Mathias-Oberti]

When clicking on a column to sort, the new sorting column isn't stored in $th.

Example :
tablesort.sort($("th.t1"))

tablesort.$th is now th.t1 element

Then, clicking on th.t2 column to sort

tablesort.$th is still th.t1 element. No update.

Proposed fix :
Store new sorted column element in $th