[8775] implement all creature difficulties + support bgs with this

former know as heroic_entry we now have 3 of those
which let us chose different creature_templates for different
map-types

normal maps will need a bit more support to use correct
spawnmodes

but for battlegrounds it works already good:
they are divided by levelrange:
    0-59  == normal spawn -> spiritguides level 60
    60-69 == difficulty=1 -> spritiguides level 70
    70-79 == difficulty=2 -> spiritguides level 80
    80    == difficulty=3 -> spiritguides level 80

this is needed mostly for alterac valley to get
right creature-templates spawned
and with that all creature->SetLevel hacks could
get removed from alterac valley code

src/game/MapManager.cpp

@@ -118,7 +118,7 @@ MapManager::_createBaseMap(uint32 id)
         }
         else
         {
-            m = new Map(id, i_gridCleanUpDelay, 0, 0);
+            m = new Map(id, i_gridCleanUpDelay, 0, DUNGEON_DIFFICULTY_NORMAL);
         }
         i_maps[id] = m;
     }