use XPath expression in erp5.js to put the focus on the good field instead of...

use XPath expression in erp5.js to put the focus on the good field instead of DOM. This is more clear, and I think more performant.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@20000 20353a03-c40f-0410-a6d1-a30d3c3de9de