Begin conversion to new widget rendering system

- Add "size" property to BaseWidget for storing actual size of widget
  - This property will be set during the measurement phase of the
    rendering process, but until that's implemented, each widget sets
    it's size manually in it's constructor
- Set a default SizePolicy in the BaseWidget constructor, and have each
  widget make a "super()" call in their constructors
- Convert each widget's render() method to return a jQuery element
  instead of an html string, for cleaner manipulation of the DOM
  - Also convert document.js -- which will likely be converted into a
    widget itself -- to accept these new jQuery elements while rendering
    the notebook ("document")