Allow changing QQmlEngine's ownership heuristics

Currently, it can be difficult to port a large Qt Script code base to QQmlEngine because of different default ownership rules.

QScriptEngine uses QScriptEngine::QtOwnership by default (similar to QQmlEngine::CppOwnership), but QQmlEngine uses a more complex heuristic to determine ownership by default.

A way to override the default heuristics can make porting easier.

Use-case

A customer's existing code is designed to pass numerous C++ objects to JavaScript via Q_INVOKABLE methods. QScriptEngine does not take ownership of these objects, but QQmlEngine does – this causes the objects to be destroyed prematurely.

The customer wants to minimize changes to their existing code by preventing QQmlEngine from taking ownership of the return values of Q_INVOKABLE methods by default.