Let user force XSD files deployment in Resources

Use case:

• Deploy a custom directory schema that was uploaded in Nuxeo Studio.

Problem encountered:

• Studio currently revolves around document schemas.
• A custom directory schema can be imported, but it will still be seen as a document schema.

Expected improvement:

• Add an option to force studio to add the XSD file in the bundle even if no document schema makes use of the XSD file.