Special XML conversion code for Firefox >= 20.0

Special XML conversion code for Firefox >= 20.0

This patch is required due to changed behaviour of the XML parser
in XmlHttpRequest that parses XML responses from the AJAX/SQL
backend.

Firefox < 20.0    <foo> was parsed and returned as <bla>
Firefox >= 20.0   <foo> is parsed and returned as &lt;bla&gt;

This breaks Icons and other HTML code in Rico.LiveGrid cells

Addendum: Chrome alias WebKit seems to behave like Firefox >= 20.0 as well