Added new creature extra flag which makes npc visible only to dead units

Port From (https://github.com/TrinityCore/TrinityCore/commit/70d5775d3f679e77c2b00418375f180f6209484d)

Source/Framework/Constants/CreatureConst.cs

@@ -208,6 +208,7 @@ namespace Framework.Constants
         Trigger = 0x80,       // Trigger Creature
         NoTaunt = 0x100,       // Creature Is Immune To Taunt Auras And Effect Attack Me
         NoMoveFlagsUpdate = 0x200, // Creature won't update movement flags
+        GhostVisibility = 0x400,       // creature will be only visible for dead players
         Worldevent = 0x4000,       // Custom Flag For World Event Creatures (Left Room For Merging)
         Guard = 0x8000,       // Creature Is Guard
         NoCrit = 0x20000,       // Creature Can'T Do Critical Strikes
@@ -219,7 +220,7 @@ namespace Framework.Constants
         IgnorePathfinding = 0x20000000,        // creature ignore pathfinding
         ImmunityKnockback = 0x40000000,        // creature is immune to knockback effects
 
-        DBAllowed = (InstanceBind | Civilian | NoParry | NoParryHasten | NoBlock | NoCrush | NoXpAtKill | 
+        DBAllowed = (InstanceBind | Civilian | NoParry | NoParryHasten | NoBlock | NoCrush | NoXpAtKill |
             Trigger | NoTaunt | NoMoveFlagsUpdate | Worldevent | NoCrit | NoSkillgain | TauntDiminish | AllDiminish | Guard |
             IgnorePathfinding | NoPlayerDamageReq | ImmunityKnockback)
     }

Source/Game/Entities/Creature/Creature.cs

@@ -813,8 +813,7 @@ namespace Game.Entities
 
             LastUsedScriptID = GetScriptId();
 
-            // TODO: Replace with spell, handle from DB
-            if (IsSpiritHealer() || IsSpiritGuide())
+            if (IsSpiritHealer() || IsSpiritGuide() || GetCreatureTemplate().FlagsExtra.HasAnyFlag(CreatureFlagsExtra.GhostVisibility))
             {
                 m_serverSideVisibility.SetValue(ServerSideVisibilityType.Ghost, GhostVisibilityType.Ghost);
                 m_serverSideVisibilityDetect.SetValue(ServerSideVisibilityType.Ghost, GhostVisibilityType.Ghost);

Source/Game/Entities/GameObject/GameObject.cs

@@ -2076,7 +2076,7 @@ namespace Game.Entities
                     bgMap.GetBG().ProcessEvent(this, eventId, invoker);
         }
 
-        public virtual uint GetScriptId()
+        public uint GetScriptId()
         {
             GameObjectData gameObjectData = GetGoData();
             if (gameObjectData != null)