#343 - do type only exports in index and model barrel file for interfaces and type aliases

[mumenthalers]

closes #343

changes:

• extend the Import class with a typeOnly flag
• accept a an optional typeOnly arg for the addImport method on the GenType class (defaults to false)
• on ModelIndex set this typeOnly flag to true when not an enum
• modifying modelIndex template to handle the typeOnly property accordingly
• modifying index template to handle the typeOnly property accordingly

--> when enums are generated with enumStyle: alias, it is not enum but type simple, therefore not causing problems.