Add [] operator to QJsonValue

QJsonValue is the return type by QJsonObject["key"] and QJsonArray[1].
If QJsonValue supports [] operator, then value retrieval sequence will be more easily chained.

Say,

the json contained in the QJsonObject p,

{
    "a": [
        {"t1": "t2"},
        {"t3": "t4"}
    ],
    "b": {
        "c": "d"
    }
}

If I want to get the "t4" value, I need to write,

p["a"].toArray()[1].toObject()["t3"].toString()

If the [] operator is added to QJsonValue, I can write it as,

p["a"][1]["t3"].toString()

which will be significantly shorter and easier to understand.

Also, it should be possible to implement since the parameter type passed to [] operator is different, one is int and one is string. And the return types can be the same QJsonValue. So there should be syntax problem.