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/Grids/Notifiers/GridNotifiers.h

@@ -1144,7 +1144,7 @@ namespace Trinity
         AllFriendlyCreaturesInGrid(Unit const* obj) : pUnit(obj) {}
         bool operator() (Unit* u)
         {
-            if (u->isAlive() && u->GetVisibility() == VISIBILITY_ON && u->IsFriendlyTo(pUnit))
+            if (u->isAlive() && u->IsVisible() && u->IsFriendlyTo(pUnit))
                 return true;
 
             return false;