WINDUP-139  Create ModelModel and fill the graph.

I would suggest adding an enum or public inner interface Props.
This
1) will keep the constants in one place:
     public interface {
            public static final String CLASS;  // or "class"? Would match the actual value.
     }
     or
     public enum Props {
           ... (constructor)
           CLASS("class");
     }
2) will keep it clear that they are all property names,
3) will prevent verbosity, since users can just type CLASS