QJSEngine/QQmlEngine: Support manipulating variadic JS functions from C++

QScriptValue ScriptContext::scriptAbortAfter(QScriptContext* ctx, QScriptEngine& eng)
{
    try
    {
        if (ctx->argumentCount() < 2)
            throw Exception(L"abortAfter(timeout, callback[, callback parameters...]) takes at least 2 parameters.");

        if (!ctx->argument(0).isNumber())
            throw Exception(L"Parameter 1 (timeout) must be an integer specifying the timeout in milliseconds.");

        if (!ctx->argument(1).isFunction())
            throw Exception(L"Parameter 2 (callback) must be a function.");

        int timeout = ctx->argument(0).toInteger();
        QScriptValue function = ctx->argument(1);

        QScriptValueList callbackParameters;
        for (int i = 2; i < ctx->argumentCount(); ++i)
        callbackParameters.push_back(ctx->argument(i));

        auto future = std::async(std::launch::async, [&]() {
            QScriptValue value;
            value = function.call(ctx->thisObject(), callbackParameters);
            return value;
        });
    }
    ...
}