[ENH] tablesorter: Now works with PluginTrackerlist. Also other related cleanup: turn off client-side processing when the server is doing it; allow url sort and filter parameters to be manipulated on the server side; change some empty tests to isset and is_null in trackerlib (wasn't filtering on the number 0).

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@46899 b456876b-0849-0410-b77d-98878d47e9d5