Create a JsonDocument/View

[narnaud]

Have a way to see/edit a Json document.

Should be fairly easily to do, see how the QtTsDocument has been integrated:

• create a JSonDocument (from TextDocument for now)
• add the type in core/settings.json
• add the type in the Type enum of Document, and the NLOHMANN_JSON_SERIALIZE_ENUM at the bottom
• update the createDocument method in project.cpp
• update widgetForDocument from mainwindow.cpp

[narnaud]

Fixed in #39