This addresses https://github.com/servo/servo/issues/3149. The immediate purpose is to support the constants in NodeFilter. The changes mostly follow the current Gecko Codegen.py. The main gist is that the generation of certain code artifacts is now gated by hasInterfaceObject() or hasInterfacePrototypeObject(), rather than by isCallback().
Source-Repo: https://github.com/servo/servo
Source-Revision: 1f9c2f9b34dede8182f45655b03380f8c26f3475