Remove Property sealed class

[manueldidonna]

Replace Property with nullable `properties. Property.Nothing would correspond to null

// before
interface Pokemon {
    val heldItem: Property<Int> get() = Property.Nothing
    val pokerus: Property<Pokerus> get() = Property.Nothing
}

// after
interface Pokemon {
    val heldItem: Int? get() = null
    val pokerus: Pokerus? get() = null
}