Update docs and code examples to GTK 4
@creiter Will GTK 4 be added to the API reference at https://lazka.github.io/pgi-docs/ at some point, or should we point GTK 4 users to something like https://amolenaar.github.io/pgi-docgen/?
@creiter Will GTK 4 be added to the API reference at https://lazka.github.io/pgi-docs/ at some point, or should we point GTK 4 users to something like https://amolenaar.github.io/pgi-docgen/?