Maint: Avoid calling observe's dispatch with named arguments

[kitchoi]

Related to #977

This PR removes the requirement on the event argument name for the dispatch callable, as the requirement unnecessarily forbids the use of other generic dispatch-like functions, such as concurrent.futures.Executor.submit(callable, *args, **kwargs). With this, it will be possible to use something like Executor.submit as the dispatch callable.
(Whether to expose this flexibility to the user API is a separate discussion though.)

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in traits-stubs

[corranwebster]

This is a good change.