Create EventHandler annotation for Proxy methods

[lidaamber]

We should add an @EventHandler annotation for Proxy methods to process incoming events in the way like this:

@EventHandler
void onSomeEvent(List<?> args);

@EventHandler("eventName")
void onSomeEvent(List<?> args);

@EventHandler({"interfaceName", "eventName"})
void onSomeEvent(List<?> args);

With this annotation the generated class for Proxy should be abstract, so that the User'll later implement handlers for methods with @EventHandler annotation.

[lundibundi]

I implemented this in 29e3ffc but I'm not sure it we should land this functionality as it can be easily achieved using a combination of Proxy and Handler (the implementation just uses Handler functions to create them here).