Further work from #352:

• Primary/SecondaryOptionalFields is only used in BibLatex and this logic should be moved into the GUI code.
• How to handle OR requirements in the GUI? Currently both are marked as required. Next to each other? Highlight required fields red if empty and green if entry hasAllRequired?
• Check for duplicate field names in requirements and optionals (use a Set? what about 'author/editor'<->'editor/author')
• Rework custom entry types and include them in the GUI 'new entry'
• What about overriding known types? (allowed? rejected? what if natbib uses 'book'? and I just need this custom type for natbib files? What about jurabib?)
• Add more tests
• Resetting overridden default entry types does not work, sometimes even crashes