I actually thought Glade was deprecated? I'm surprised to see it mentioned... Or maybe I'm wrong or maybe the author didn't know!
Until then, you can either avoid UI files, write them by hand, or my favorite option: use blueprints[1], a file format that compiles to UI files. For instance, this file[2] describes a form with a folding advanced section, complete with functions to call when buttons are pressed, and CSS class names to apply to widgets.
[1]: https://jwestman.pages.gitlab.gnome.org/blueprint-compiler/ [2]: https://github.com/JMS55/whatthefn/blob/master/res/blp/profi...