Move Event wrapper and its associated traits and impls to a separate module

[luukvanderduim]

Could we move the Event wrapper plus associated traits and impls to a separate module - and gate it?

Rationale is that we have a lot of event-related traits and implementations.

• It is tidier to have the bits pertaining the wrapper in a (eg.) 'event_wrapper' module.
• AT users would likely use the trait, 'atspi types-only' users won't.
  Having a gate hiding/exposing the feature might help the latter user a little.

[TTWNO]

Good idea!