Refactor entry type/custom entry type logic

[stefan-kolb]

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

[koppor]

Regarding "OR": In the doc at "Either/or fields", it is talked about that this should work with a special configuration. At least for custom types.

[tobiasdiez]

Also:

• Store custom entry types in MetaData class and reuse some of the parsing/writing logic.

[matthiasgeiger]

• "Rework custom entry types and include them in the GUI 'new entry'" is already included:
  

• 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?)

  • Already supported: Global redefinition of standard types
  • Will be supported with Improve entrytype customization #2331: distinction between bibtex and biblatex
  • Not possible: Overwrite more specifically

• Resetting overridden default entry types does not work, sometimes even crashes

  • works for me - but this requires time as all entries in all open databases are rechecked (see also Customizing the entry types freezes Jabref #2318)

[LinusDietz]

superseded by https://github.com/JabRef/jabref/projects/6