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. 
What about tests for the other methods that were added, `_elementsFromClassKey` and `_classesFromObject`?