Allow specifying fetches for relations in entity views

[beikov]

Contrary to what I thought was possible with JPQL when creating #146 it actually is possible to define fetches for sub-relations. When #190 is done, we can also support doing fetches in entity views. I propose to add a new method String[] fetches() default {} to Mapping that can be used to specify fetches for plain entities or embeddables.
Also update the documentation about entity mappings when this is done.