Blueprints: InspectorEntityAttribute has to get a blueprint via a blueprint id

[coeing]

Because of some refactoring and cleanup, the EntityManager now doesn't have a reference to the BlueprintManager anymore, but they are completely separate.

The InspectorEntityAttribute needs the BlueprintManager though to map its blueprint id correctly and create an entity out of it. Right now the inspector attributes only have access to the EntityManager. Some kind of DeserializationContext should be given to the attributes, so they can correctly deserialize themselves.