Use project specific enums in entities

[witchpou]

Please refer to Create, update and persist enums with projectbuilder for an introduction on project specific enums and wait with the implementation of this issue until the mentioned issue was implemented.

Now that project specific enums can be created on the projectbuilder, they should be addable to entities. Right now, enums are just added with the type "Enum" in the projectbuilder, which will lead to compilation problems. Instead, one of the previously created enums should be chooseable here.

Acceptance criteria:

• Project specific enums can be added as properties for entities in frontend
• Project specific enums are recognized in project generation and can be used in Freemarker templates