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
Now, of course, this is repetitive. I wonder if these lines should be abstracted out so that e.g. `on_add()` is just `_set_callback(self, '_add_callback', callback)`.