json: Add defaultValue to QJsonValueRef toInt/toBool/toDouble/toString

Currently QJsonValue and QJsonValueRef behave differently in
regard to the default values leading to confusion compile errors
depending on which of the two types one is actually using. Before
this change it was possible to write:

QJsonValue value = jsonObject["item"];
QString name = value.toString(QStringLiteral("default"));

but not:

QString name = jsonObject["item"].toString(QStringLiteral("default"));

Change-Id: Id1185acf339aa3a91e97848e85d068f84552df71
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>