[#3560] - Description show at the table header not at a table cell (com_installer)

See:
http://issues.joomla.org/tracker/joomla-cms/3560
and here:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30564&start=0

This PR is a rewrite against staging of this PR by @drmmr763 agianst master: https://github.com/joomla/joomla-cms/pull/1759