Add convenient JSON API to modify documents

The current API appears to make modifications to values instead of the
document itself. This makes it quite cumbersome to modify a document.
For example, to change the value of 5 to 10 in the json "[1, 2, [3, 4,
[5, 6]]]", one has to write the following code.

QJsonDocument doc = QJsonDocument::fromJson("[1, 2, [3, 4, [5, 6]]]");
QJsonArray a1 = doc.array();
QJsonArray a2 = a1[2].toArray();
QJsonArray a3 = a2[2].toArray();

a3[0] = 10;
a2[2] = a3;
a1[2] = a2;

That is to change a value somewhere in the document, one has to update and then update all the way up.