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

AI/BattleAI/BattleEvaluator.cpp

@@ -686,7 +686,7 @@ bool BattleEvaluator::attemptCastingSpell(const CStack * activeStack)
 		spellcast.spell = castToPerform.spell->id;
 		spellcast.setTarget(castToPerform.dest);
 		spellcast.side = side;
-		spellcast.stackNumber = (!side) ? -1 : -2;
+		spellcast.stackNumber = -1;
 		cb->battleMakeSpellAction(battleID, spellcast);
 		activeActionMade = true;