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
The phrasing here is a bit confusing. By "cancel" do you mean remove/delete the callback?