Support for per-widget MVC extension, refs #16594.

Automation ran clean with FF-latest, Chrome-latest.
To pass automation with IE8 I had to do:

* Remove [changeset 30781](http://trac.dojotoolkit.org/changeset/30781/dojo), as IE8 does not have `element.addEventListener()`
* Needed to explicitly import `dojo/ready` module in some of mobile tests (Some `dojox/mobile` module seem to rely on `dojo.ready()` which needs `dojo/ready` to have been loaded beforehand)