modello:java: Support exclusion of individual classes

[kwin]

Sometimes only some classes being generated out of a Model should be used.
Currently it is not possible in the java goal to generate only some classes of the underlying model(s).

I propose to add a new parameter to https://codehaus-plexus.github.io/modello/modello-maven-plugin/java-mojo.html which allows to exclude emitting beans for certain classes.