All: Classes Option

This replaces PR #790 It updates the widget factory to handle the classes option via setOption, and updates all the widgets to use the `_elementsFromClassKey` method to set what elements belong to what classkeys for programmatic updating. It also updates all classes options to use `""` instead of `null` to make it easier to concant new classes. 
Shouldn't "ui-widget-content" also be moved to the `classes` option? Its styles are defined in `theme.css`. Applies to many other widgets that use this class, too.