"New X10 Class" wizard is inappropriately Java-centric

The "New X10 Class" wizard extends the Java "New Class" wizard, which inappropriately exposes various pieces of Java-centric information that either doesn't make sense for X10 at all, or only makes (some) sense when using the Java back-end.

We may want to add support for X10-specific features here too, e.g. for defining properties, class invariants, type parameters, and so on.