JavaGenerator should allow for overriding POJO constructor generation

[lukaseder]

Getters and setters generation and other things can already be overridden in the JavaGenerator, but not POJO constructor generation. In POJOs, there's no need for calling jOOQ-specific super constructors, so the generated constructors don't have to be under jOOQ's control. Besides, it is possible to define a super class for POJOs, so it should be possible to override the constructor generation anyway.