Add QJsonValue::toInt().

It's a nice feature to have.

MSVC also complains about using doubles to create enum values, so
the ugly workaround is:

enumValue = MyEnum(qRound(json["myEnumValue"].toDouble()));

[ChangeLog][QtCore][QJsonValue]Added QJsonValue::toInt().

Change-Id: I1a200b912abf66b2e96390b1980caff26cfa2685
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>