Introduce a Config subclass for Entities

[b1rger]

the _search_fields variable already led to confusion and there is also the wish to be able to store human readable metadata in the classes (a description of the entity that is used in the documentation)

We could use a subclass for that, similar to the Django Meta class

[sennierer]

Decided to use a APIS inner class for adding configurations and documentation strings that are APIS specific.
Inheritance needs to be still sorted out.

[sennierer]

inner classes are inherited. so we should find a similar solution as django does with model classes by removing abstract=True