Missing explenation in subclassing

[jakubGodula]

Section 5.2 Subclassing does not mention adding the following code in mod.rs.

impl Default for CustomButton {
    fn default() -> Self {
        Self::new()
    }
}

The first part works well without it, but it is required by Adding functionality part.
I know that it is not GTK-related and that it is common knowledge among programmers that it is required, but I am sure that mentioning it will make it easier for newcomers to follow along with this book.

[bilelmoussaoui]

Patches would be appreciated, as you are the one knowing best what is missing and where at this point