Use this to annotate an event-handling method that should be registered in
the proxy rather than being registered in the presenter. Handlers annotated
in this way should not be registered in the Presenter.
The presenter will be instantiated as soon as the proxy intercepts the event, so
the presenter will handle the event even if it was not yet initialized.
Methods annotated by @ProxyEvent must return void and accept
a single parameter derived from GwtEvent.
This event class must have a static getType method returning a type
derived from Type.