Add interface to add sub entities to an entity

To add for example a new detail to a lecture, use the new AddToEntity
mechanism. The class is a SelectEntity (see
22041f89df420c510db884f94b7142177b0169d1) and transports new entities to
add to the result of the SelectEntity.
Neccessary resources have a path to add a new entity to this one.

Use the KeyReader for easier access.