[FactionInfo] Expose getter and setter for faction relationships

[oznogon]

• Add conversion functions for EFactionVsFactionState as a parameter and return value

• Replace the overloaded FactionInfo::getState function with separate getRelationshipWith and getRelationshipBetween functions, and replaces usage of FactionInfo::getState with the equivalent getRelationshipBetween

• Expose getRelationshipWith to scripting:

  human_navy = getFactionInfo("Human Navy")
  kraylor = getFactionInfo("Kraylor")
  human_kraylor_relationship = kraylor:getRelationshipWith(human_navy) -- returns "enemy"
  

• Add setRelationshipWith

Towards #530.