feat (AdaptiveLevel) : cap player's level on area's level.

When player enter an area, it's level is capped.
Only the stats are capped. The level displayed is still the same, so equipment is still compatible and the player doesn't lose its talents.
What is impacted :
 - Visibiliy of low-level quests
 - Quest XP reward
 - Discovery XP reward
 - Creatures aggro