Core/Misc: Fixed player corpse looting, added player corpse loot and some more

Port From (https://github.com/TrinityCore/TrinityCore/commit/89f728cd5b6ae3041fdae3c853ac4b0ac5f0a848)

Source/Game/BattleFields/Zones/WinterGrasp.cs

@@ -567,7 +567,11 @@ namespace Game.BattleFields
                 return;
 
             if (victim.IsTypeId(TypeId.Player))
+            {
                 HandlePromotion(killer, victim);
+                // Allow to Skin non-released corpse
+                victim.AddUnitFlag(UnitFlags.Skinnable);
+            }
 
             // @todo Recent PvP activity worldstate
         }

Source/Game/BattleFields/Zones/WinterGraspConst.cs

@@ -548,7 +548,7 @@ namespace Game.BattleFields
         public const uint Max = 7;
     }
 
-    struct WintergraspAreaIds
+    public struct WintergraspAreaIds
     {
         public const uint WintergraspFortress = 4575;
         public const uint TheSunkenRing = 4538;