Chapter 9 missing derived_properties and subclassing

[almindor]

Bug description

Chapter 9: List Widgets section in the book is missing the subclassing section required for the code to even compile (e.g. impl ObjectImpl for IntegerObject {}

#[glib::derived_properties] is also never mentioned so even if people figure out the code required from the subclassing section they will be greeted by a runtime panic because #[glib::derived_properties] is not in Chapter 5.2 either.