(bug 35526) Make jquery.tablesorter use a stable sort

(bug 35526) Make jquery.tablesorter use a stable sort

In r86337, jquery.tablesorter was changed from using the standard
Javascript Array.sort to a custom merge sort, with the justification
that it eliminates an eval and merge sort is stable. However, the
implementation used is not, in fact, stable, and making an in-place
merge sort stable reportedly kills performance.

Instead, let's just go back to using Array.sort, but with a closure
(basically the same comparison function used by the merge sort) rather
than an eval and using the already-calculated "position" as a tiebreaker
when two rows are otherwise equal to make it stable.

Change-Id: Idc50127d3bfec2b1727f397a9780b359fd56055e