[qml, js] Symbol-identified properties and methods for QMetaObject-based entities

There is no way to define Symbol-identified properties and methods for Q_OBJECT and Q_GADGET objects, propagated to qml-js for using from qml. For concrete example - Symbol.iterator is exclusively used inside the js-engine to fetch iterator from an iterable objects, so, there is no way to use a QObject or gadget as iterable.

Suggestion: add a way to define Symbol-identified properties and methods for Q_OBJECT and Q_GADGET objects, propagated to qml-js.

Syntactically that can be done through

• Q_CLASSINFO or
• invocable method tag,

plus the appropriate support from qml and js engines. Example

class MyGadget
{
    Q_GADGET;
    Q_CLASSINFO("@@Symbol.iterator", "myIteratorMethod");
public:
    Q_INVOCABLE MyIterator myIteratorMethod();
};

//or
class MyGadget
{
 Q_GADGET;
public:
 Q_JS_ALIAS("@@Symbol.iterator") Q_INVOCABLE MyIterator myIteratorMethod();
};

Another way - provide an ability to register a programmer-defined js-prototype (QJSValue or similar js-object) for concrete Q_OBJECT/Q_GADGET c++ types. These prototypes will be automatically settled as prototypes for converted objects, instead of (or as a base of)  QV4::ExecutionEngine::ValueTypeProto (and similar for Q_OBJECT). Example

// programmer builds js-prototype object
QJSValue myGadgetProto = jsEngine->evaluate("let res = {}; res[Symbol.iterator] = function() {/*blabla..*/}; return res");

//and registers it for a some value-type as prototype
qQmlRegisterValueTypeProto<MyGadget>(myGadgetProto);

// and for qobject-based
qQmlRegisterQObjectDerivedProto<MyQObjectBased>(myProto);

//... qml-js engine will use provided prototypes for wrapped gadgets/qobjects

Or some similar..