Support for observe in ExtensionPoint

Traits 6.1 introduces a new trait notification/observation framework: `observe`. The mini-language for this framework has changed for observing mutations in a container such as a list.

Currently `ExtensionPoint` emits property change event using a trait named _name_ and _name_items_. This allows users to use `on_trait_change` as if an `ExtensionPoint` was a `List` and listen to "mutations" to it even though the list is never cached. In other words, it behaves as if it was two `Property` combined. Changing `on_trait_change("name_items")` to `observe("name:items")` won't work as expected. This issue is about how to make `ExtensionPoint` be compatible with `observe`.

I might be able to put together a small example and post here later.