Correct base types management in AssetModel

Summary

The DataFormat base types provided through AssetModel shall not be used everywhere a DataFormat is used.

An analysis must be done to determine where exactly it make sense to have them and also if they must be made availble together with the other Dataformats provided by the prefix.

AssetModel shall be purged from these types and a dedicated subclass created if it makes sense to provide them all in one or more places in the code.

Depending also on whether these base types should be the only type used, another model might be created or QStringListModel used directly in these cases.