Don't initially disable bosses
Created by: christopho
Bosses are initially disabled when they are created on the map. This special case should be removed because bosses should be like any enemy. The user can easily disable an enemy if she wants. Furthermore, this is error-prone because most users are not aware that bosses is initially implicitly disabled, and forget to take care of that in enemy:on_created() (that gets called anyway).
If enabling/disabling enemies is an explicit choice, understanding what happens will be more natural.
And for the same reason, enemy:create_enemy() should not forward the "disabled" property. This is error-prone too because enabling the first enemy later does no longer acts on the child.