Add new domain of application to skills

[Nando Neck]

Some advantages or special abilities add additional domains of application to skills. It would be nice to list active and inactive domains at the skill info.

This would need some additional fields for those spabs and advantages like:

additional_domain: [
    "sinnesschaerfe": "Fuchssinn"
],
"name": "Fuchssinn"

This would also enable further Fertigkeitsspezialisierungen