Add `@Body`/`@Initializer` annotations

[dan-leech]

I like new @Assert decorator very much. But I also need @Event decorator. :)

I think it would be very good to have custom decorators. Might they exist? I expect something like JsonConverter but for a constructor initialiser with overriding init method.

In generated code it would looks like this:

class Example {
  Example(Params params): assert(someAssertion  == true) {
    @Initialiser().init?.call() // or better: call(params)
  }
}

[rrousselGit]

What would an @Event do?

[dan-leech]

For events I need to send custom events like "model has been created" to a broadcaster, but this is custom code too.

What would an @Event do?

It's just the same idea as for @Initialiser witch can be customised as developer want. The idea is running any code in a class constructor body.