[FEATURE] (Validation): Context-specific validation through Validation Groups
Sometimes, Validation should only be executed on Persist, or on the Controller
invocation. Especially when building multi-step forms, one needs to be able to
partially validate an object and store the invalid object in the database.
This change set implements "validation groups", i.e. each validator annotation
can have a "validation group" assigned, and when validation happens, the validators
get only executed if at least one validation group matches.
Furthermore, it adds a documentation skeleton for partial validation.