Add conversion between QVariantHash and QJsonObject

Adds automatic conversion from QVariants with a QVariantHash to
QJsonValue and explicit methods for conversion between QVariantHash
and QJsonObject.

[ChangeLog][QtCore][QJsonObject] Added conversion to and from QVariantHash

Change-Id: I140ef881463acabaab2648e28209487d8ad18e0d
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Be able to read and write properties to Q_GADGET

Change-Id: Ic12f465d31459748ca08ac8c457fd61a5773e2e2
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Make Q_INVOKABLE work on Q_GADGET

Methods can be invoked with QMetaMethod::invokeOnGadget

Change-Id: Id734868bb530b02587daf0f62bce01798ade2ac2
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>