#1423: add GraphQLResult and wrap successful result

[t1]

Also: rename Wrapper#isNotEmpty to Wrapper#isNonNull and fix some warnings

TODOs:

• [ ] Map all declared exceptions to fields
  • [ ] In the schema
  • [ ] When executing
• [ ] Consider @GraphQLResult annotations on an API class
• [ ] Consider @GraphQLResult annotations on a package
• [ ] Provide a config option to globally enable result-wrapping (and add to GraphQLResult javadoc)
• [ ] Maybe consider ErrorCode annotation (but convert kebap to camel?)
• [ ] Documentation

[t1]

@phillip-kruger: I have several smaller problems that you might be able to help with.

1. The main branch doesn't compile (GraphQLWSSubprotocolHandler and GraphQLTransportWSSubprotocolHandler)
2. In the main branch, the ReferenceCreator causes a StackOverflow
3. I've added a TCK test for result, but it fails because it doesn't find the order method, but the same request runs in the runner!

I could fix 1. and 2., but I don't understand how they got in there.