Improve/redesign models in QML

The current implementation of models in QML is suboptimal, we should have an implementation that:

• is typed like GraphQL, Avro, protobuffers, Parquet,... i.e.
  • types consisting of basic types (int,double,string,binary data, null, (empty?))
  • lists of a uniform type (list<type>)
  • maps of a uniform type (map<string,type>)
  • objects (list of fields(text) each field having a type)
• easy interconversions from/to json and similar formats
• provides a way to register observers for various notifications (local changes, changes should not propagate up the hierarchy by default).
• an API that can accommodate data that can be loaded and saved asynchronously

This is something that can make the building of an application talking to a RESTAPI, or any data oriented application much simpler.

This could be prototyped in Ultralite first.