EntryConver: Simplify prototype and possible values

Is your feature request related to a problem? Please describe.
Currently prototype keys are written to current value and must match entries in Value.Possible. This requires a lot of implicit knowledge and increases complexity because code is distributed over several places that all need to fit together. This happened, because prototypes were introduced as an addition to the Entry-format later.

Describe the solution you'd like
Possible values should only be relevant for primitive values. For classes and collections we use Prototypes and we should only use them. Instead of reading possible values and finding a match among the prototypes, we could directly populate from prototypes. I would also like to write prototype key to its identifier, which is currently "CREATED" and overwrite with "CREATED" when the prototype is instantiated.
The AddPrototype should also use the Identifier first and use the current behavior as a fallback, just like we keep possible values populated for this major.