Improve model of skirmish traits

[dmorcellet]

See for instance ID=1879154340

• trait prerequisites: "Trait_Prerequisites_Logic"
• pet effects: "Skirmish_TraitEffect_PetMods"
• static icon overlay: "Trait_Static_Icon_Overlay"
• rank overlay progression: "Trait_Rank_Overlay_Progression"
• trait sub-category (optional), and associated enum: "Trait_Sub_Category"

[dmorcellet]

Done: static icon overlay and rank overlay progression (and related icons).

[dmorcellet]

Effects are found in EffectGenerator_TraitEffectList too.

[dmorcellet]

Done: pre-requisites logic and sub categories. TODO: effects.

[dmorcellet]

Pre-requisites are now displayed. Effects data is recorded in the dev notes. Will do it in another issue. Closing this one.