Add type parameter to ListModel and ListStore

[V02460]

Each instance of ListStore and ListModel works on a concrete and fixed item type. Currently this is handled by checking and passing around a glib Type value. In Rust this can be expressed more elegantly by a type parameter. The big advantage is that there is no more fallible casting when using the different methods on these instances and Rust’s type system can be leveraged for e.g. type inference.

Before

struct ListStore;

fn new(item_type: glib::types::Type) -> Self;
fn append<P: IsA<glib::Object>>(&self, item: &P);
…

After

struct ListStore<T: IsA<Object>>;

fn new() -> Self;
fn append<P: IsA<T>>(&self, item: &P);
…

[sdroege]

You'd need to add some API for casting between ListModel<T> and ListModel<U> too then, and either needs to make sure that the type is a supertype (not subtype!) of the model's item type.

I think this would improve the API considerably but it also means that the bindings can't be autogenerated anymore (nothing where ListModel or ListStore appears in the API).