book: Don't use resources for composite templates

[sophie-h]

https://gtk-rs.org/gtk4-rs/stable/latest/book/composite_templates.html

This chapter is rather complex and requires unpleasant things like writing a build.rs. As far as I can tell, this could all be avoided by just using

#[template(file = "window.ui")]

The .ui-files could just be stored in the same folder as the source, as many apps and libraries do. Since they are often edited together, it makes navigating code much easier.