Include project layers in JSON export even if there are no annotations on that layer

[reckart]

Is your feature request related to a problem? Please describe.
The UIMA JSON CAS export includes an optimization where type definitions from the type system section are omitted if there is no annotation of the corresponding type in the CAS. Consider the following scenario:

• user creates a project in INCEpTION
• uploads a document and then exports it as JSON CAS
• then loads it in Cassis and tries to add an annotation using one of the layers from the project
• at this point the type is not found in the exported CAS because no annotation exists in INCEpTION yet

Describe the solution you'd like
The UIMA JSON CAS export should always include all the types that belong to the INCEpTION project from which it was exported. However, unused DKPro Core types which also float around in the system should still be omitted from the type system section of the JSON unless corresponding annotations exist in the CAS.