(bug 40044) make $.tablesorter treat alt attribute like text

Now images in cells will not be ignored when sorting; instead, their alt
attribute will be used for the sort key.

Internal interface changes:
* renamed getElementText() to getElementSortKey() with changes

Change-Id: I3e9898f3424a995507095384e80c8b570da118f0
Merge "(bug 40044) make $.tablesorter treat alt attribute like text"