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/game/Entities/GameObject/GameObject.cpp

@@ -1598,7 +1598,7 @@ void GameObject::CastSpell(Unit* target, uint32 spellId)
     Creature *trigger = SummonTrigger(GetPositionX(), GetPositionY(), GetPositionZ(), 0, 1);
     if (!trigger) return;
 
-    trigger->SetVisibility(VISIBILITY_OFF); //should this be true?
+    trigger->SetVisible(false); //should this be true?
     if (Unit *owner = GetOwner())
     {
         trigger->setFaction(owner->getFaction());