programming model for converters and listeners

[gavinking]

The spec does not say that:

1. entity listener classes must be non-abstract, nor
2. whether they must be public.

It does say that they must have a public no-args constructor. I doubt this is even correct when they're instantiated via CDI.

Even worse, the spec does not say much at all about converters:

1. it doesn't mention they can't be abstract,
2. it doesn't say anything about constructors,
3. it doesn't mention anything about visibility.

We should clear all this up.