There seems to be a new file format in the works to define Gnome UI with a lot of advantages:

• Concise syntax. No more clumsy XML! Blueprint is designed from the ground up to match GTK’s widget model, including templates, child types, signal handlers, and menus.
• Easy to learn. The syntax should be very familiar to most people. Take a look at the reference to get started.
• Modern tooling. Blueprint ships a Language Server for IDE integration.