- Always unescape element contents on webdav.xmltools

      - Use saxutils to escape/unescape values for/from
        PROPFIND/PROPPATCH.

      - Make OFS.PropertySheet use the escaping function from
        webdav.xmltools.

      - Escape/unescape " and '

      - Set a default value of '' for the new 'alt' property as not to
        break existing content.