JsonDocument: editing the document won't update the internal json representation

[narnaud]

Editing the document in Knut's text view won't update the m_jsonData.
These changes therefore won't be reflected in the value/hasValue calls. setValue will also override any changes to the document that weren't saved.

[narnaud]

One possible workaround would be to save everytime you use JsonDocument API, but I'm not sure that's the best way.