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/Creature/Creature.cpp

@@ -814,7 +814,7 @@ bool Creature::Create(uint32 guidlow, Map *map, uint32 phaseMask, uint32 Entry,
     }
 
     if (Entry == VISUAL_WAYPOINT)
-        SetVisibility(VISIBILITY_OFF);
+        SetVisible(false);
 
     return bResult;
 }