QJsonObject::iterator cannot be subtracted, as required for random-access iterators

QJsonObject::iterator has iterator_category defined as std::random_access_iterator_tag, but does not implement the requirements for random-access iterators defined by the C++ standard. This causes many standard library algorithms to fail.

Specifically, given iterators a and b, the expression `a - b` must be valid, and return the distance between the two iterators. (see http://en.cppreference.com/w/cpp/concept/RandomAccessIterator)

Because the iterators promise to conform to the random access iterator concept, but don't actually implement the required interface, any code using standard library algorithms such as std::all_of fails to compile.

According to the class' documentation, the class was (correctly) tagged as a bidirectional iterator prior to 5.6. Perhaps that change should simply be reverted.