make close-and-halt work on new tabs in Chrome

this is not possible in Firefox, as new tabs/windows which were *not*
opened via a script ( window.open call ) are not allowed to be closed
via window.close and will yield a message like

--
[11:50:59.691] Scripts may not close windows that were not opened by script. @ http://localhost:8888/static/notebook/js/menubar.js:105