Fix a bunch of bugs with the new Soul Steal mechanics

- The description didn't reflect the behavior of using the max HP instead of the
  current HP.
- For that matter, it also didn't use the right amount, it used the base max
  instead of the vitality max, so it didn't steal the amount that was actually
  advertised.
- Kills made with Soul Steal are now correctly credited to the player, and
  therefore count toward missions (Fair enough, since it uses far more EP now).
- Soul Steal deals the damagetype "LifeSteal", allowing monsters to react
  differently to it if they wish.
- The player is only healed for the amount that is actually stolen from a
  monster, which can be less than the attack % if the attack kills said monster.