Allow connecting signals with builders

[pozix604]

A major benefit of builders is to configure an object "in one go". It'd be nice to be able to set up signals for widgets using builders:

    let button = Button::builder()
        .label("Press me!")
        .clicked(|b| b.set_label("Hello World!"))
        .build();

instead of what we have to do now:

    let button = Button::builder()
        .label("Press me!")
        .build();

    button.connect_clicked(|button| {
        button.set_label("Hello World!");
    });

(from https://gtk-rs.org/gtk4-rs/stable/latest/book/widgets.html)

[A6GibKm]

The idea behind a builder is to construct an object, properties are interesting to set during object construction and some cannot be set afterwards. However signals are always after it has been created.