Core/Units: Rename and change Get/SetVisibility to use a bool value instead of an unnecessary enum

Also replace some SetVisibility hacks to directly call UpdateObjectVisibility

--HG--
branch : trunk

src/server/scripts/Northrend/Naxxramas/boss_sapphiron.cpp

@@ -100,7 +100,7 @@ public:
             float x, y, z;
             me->GetPosition(x, y, z);
             me->SummonGameObject(GO_BIRTH, x, y, z, 0, 0, 0, 0, 0, 0);
-            me->SetVisibility(VISIBILITY_OFF);
+            me->SetVisible(false);
             me->SetFlag(UNIT_FIELD_FLAGS, UNIT_FLAG_NON_ATTACKABLE);
             me->SetReactState(REACT_PASSIVE);
 
@@ -344,7 +344,7 @@ public:
                             EnterPhaseGround();
                             return;
                         case EVENT_BIRTH:
-                            me->SetVisibility(VISIBILITY_ON);
+                            me->SetVisible(true);
                             me->RemoveFlag(UNIT_FIELD_FLAGS, UNIT_FLAG_NON_ATTACKABLE);
                             me->SetReactState(REACT_AGGRESSIVE);
                             return;