Add EventfulList and EventfulDict trait types.

**Lots** of useful applications in Widgets.

The events are designed to catch all of the modifying methods of the list and dict objects.  The following methods exist to register handlers:

EventfulDict members:
- on_add
- on_del
- on_set

EventfulList members:
- on_insert
- on_del
- on_set
- on_reverse
- on_sort
Well one argument for not allowing multiple callbacks is that with the current design its very clear how events can be canceled, by returning `True`.  With multiple callbacks, it's not as straight forward, but still possible.  We'd need to capture all the returns and check `any(results)`.