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/Outland/CoilfangReservoir/SerpentShrine/boss_lurker_below.cpp

@@ -141,7 +141,7 @@ public:
                 pInstance->SetData(DATA_STRANGE_POOL, NOT_STARTED);
             }
             DoCast(me, SPELL_SUBMERGE);//submerge anim
-            me->SetVisibility(VISIBILITY_OFF);//we start invis under water, submerged
+            me->SetVisible(false);//we start invis under water, submerged
             me->SetFlag(UNIT_FIELD_FLAGS, UNIT_FLAG_NOT_SELECTABLE);
             me->SetFlag(UNIT_FIELD_FLAGS, UNIT_FLAG_OOC_NOT_ATTACKABLE);
         }
@@ -189,7 +189,7 @@ public:
                 {
                     if (Submerged)
                     {
-                        me->SetVisibility(VISIBILITY_ON);
+                        me->SetVisible(true);
                         Submerged = false;
                         WaitTimer2 = 500;
                     }