Add JSON validation through schemas

Squish uses JSON Graph Format (JGF) through QJson for its new Model-based Testing feature. There is a schema available for JGF that we could use to validate most parts of a model or testcase file used with Squish. Some requirements can't be expressed with schemas.

While having schema validation in Qt would be pretty nice to have, there's the additional caveat for Squish that we're stuck to rather old Qt versions (currently 5.9, upgrade to 5.15 is planned).