jquery.tablesorter: Optimise getElementSortKey() code

jquery.tablesorter: Optimise getElementSortKey() code

* Document why this uses $().data() instead of $.data(),
  as the rest of this file does.

* Move single-use $() to where it is needed.

* Use native Image.alt directly instead of going via jQuery
  and attributes. This also obviates the need for a default,
  as the DOM handles that already.

* Remove needless $.makeArray(), use $.map() directly which
  supports the same array-like objects as $.makeArray().

* Use native elem.textContent directly for text nodes,
  instead of $(elem).text().

Change-Id: I92da50e1c01f11585d2bac4f01dba1f671444a63