Source Book localization

[NoxMortem]

Different language Source Books should be handled as subnodes which are suggested as source books depending on the language the localization is for. See #166

Instead of adding one Source Book per language it should be

Shadowrun 5th Edition
-> Localization (de, Shadowrun 5te Edition)

This sort of localization nodes probably need a change in the model as they do not only replace the name but also the cover, etc.