No convenience functions for nested CBOR maps

This suggestion comes from this post on the forum. Please read it for a full understanding of the issue.

Basically, if one is using a QCborStreamWriter to save data, there is no easy way to create nested maps or array, especially with deep hierarchies. Indeed, QCborStreamWriter::append does not handle QCborMap/QCborArray as argument.

It would be much easier if one could just create a low level QCborMap, insert it in a higher level QCborMap and so on until finally it is written to the file with an append() in the writer.