Make the Web IDL valid

[domenic]

E.g. subscribe() is missing a return type, many parameters are missing names.

[domenic]

Another invalidity: using void instead of undefined.

Other potential improvements, not necessary for validity but maybe good:

• Use VoidFunction to eliminate ObserverCompleteCallback
• Use more specific types than Function in the operators, e.g. something like callback MapperCallback = any (any value, unsigned long long index).