Create EventHandler annotation for Proxy methods

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

```java
@EventHandler
void onSomeEvent(List args);
```
```java
@EventHandler("eventName")
void onSomeEvent(List args);
```
```java
@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.