popup.js: use native methods rather than dojo/dom*, except for dom-geometry.position()
uacss.js simplifications

1. Remove setting of box-model CSS class.  This code existed in Dojo V1 mainly for IE
    running in quirks mode.   In modern browsers, everything is content-box model,
    unless you set CSS to override that.   But the CSS for border-box sizing can
    be put on any node, not just <html> or <body>, so I'm not sure it makes sense to
    try and detect it.

 2. Remove code for checking if document is in RTL mode.   I think this can be done through direct
     CSS, i.e. "[dir=rtl]".
Widget.js: use native methods rather than dojo/dom*.