[FIX] tablesorter: only set shortcut variables once and use parent static in child classes; don't repeat column-specific code for each column when rendering header; support external filters; better handling of ids

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@48007 b456876b-0849-0410-b77d-98878d47e9d5