Consider switching to blueprint for UI files

[lo2dev]

Blueprint is a markup language designed for GTK apps that compiles to XML.

It offers a more pleasant and easier to read syntax compared to XML (in my opinion).

More details here: https://jwestman.pages.gitlab.gnome.org/blueprint-compiler/

It's really easy to learn and the porting process shouldn't be too bothersome as there isn't much to port. I'm willing to port it over if you want, fixing some issues in the process.

[gianni-rosato]

If you're willing to open a PR with a port, I'll happily merge it!