Unified handling of battle sides ID's

- Replaced BattleSide namespace-enum with enum class
- Merged two different BattleSide enum's into one
- Merged BattlePerspective enum into BattleSide enum
- Changed all places that use integers to represent battle side to use
BattleSide enum
- Added BattleSideArray convenience wrapper for std::array that is
always 2-elements in size and allows access to its elements using
BattleSide enum

lib/battle/CPlayerBattleCallback.cpp

@@ -76,7 +76,7 @@ const CGHeroInstance * CPlayerBattleCallback::battleGetMyHero() const
 
 InfoAboutHero CPlayerBattleCallback::battleGetEnemyHero() const
 {
-	return battleGetHeroInfo(!battleGetMySide());
+	return battleGetHeroInfo(otherSide(battleGetMySide()));
 }