_Widget - include a placeAt method for all widgets

[dante]

I've created a function (placeAt) that, when mixed into _Widget provides all widgets with a dual-purpose dom-placement utility. First, it wraps the dojo.place API, allowing you to automatically place the widget's domNode somewhere in the DOM. Next, you can pass a widget reference instead of a domNode reference, and the passed widget has an addChild method, the widget is added to to the passed widget. For added fun and convenience, placeAt returns 'this', to allow for chaining and/or storing of the same object ref.

[dante]

(In [14602]) fixes #7298 - add a placeAt method to _Widget for simple (and chainable) dom manipulation on a per-widget basis. !strict

[dante]

(In [14603]) refs #7298 - optimize some. dojo.place passed reference through dojo.byId, and undefined == "last", so no need for the

on position. thanks les. !strict