In GTK, a common scheme is to let a function creating a specific widget
to return a `GtkWidget *`, rather than the specific subtype, since you
often need to call API of GtkWidget, avoiding some useless casts.

For bindings however (and especially bindings to compiled languages),
this is a bit annoying, as you have to explicitly change the type of the
return value (downcast), which is not trivial (or at least desirable) in
each language.

Luckily, we can use `(type ...)` annotation for this use case, leaving
the C API unchanged, while improving the experience for bindings.