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