SystemLanguagesFieldDefinition builds options in getter, using MgnlContext.

Making definitions return contextual info (e.g. currentLocale) defeats the purpose of having a single, valid, reliable configuration for all, and must be avoided. It might lead to errors when accessing definitions outside of any Context (as occurred in linked issue).

• Definitions should always be stupid POJOs.
• The logic in #getOptions() should be moved to a custom SelectFieldFactory, overriding #getSelectFieldOptionDefinition()
• The FieldTypeDefinition should be registered in in JCR config, under security-app module.