Core/AI: Factory functions cleanup

- Get rid of fugly void pointers
- Streamlined AI selection code, using Permissible as it should've been instead of harcoded checks on the selector code
- Moved IdleMovementGenerator singleton to factory code

src/server/game/Entities/GameObject/GameObject.cpp

@@ -144,12 +144,9 @@ bool GameObject::AIM_Initialize()
     return true;
 }
 
-std::string GameObject::GetAIName() const
+std::string const& GameObject::GetAIName() const
 {
-    if (GameObjectTemplate const* got = sObjectMgr->GetGameObjectTemplate(GetEntry()))
-        return got->AIName;
-
-    return "";
+    return sObjectMgr->GetGameObjectTemplate(GetEntry())->AIName;
 }
 
 void GameObject::CleanupsBeforeDelete(bool finalCleanup)