Complete port to GTK4 + massive code cleanup (fork revival)

[j4imefoo]

This pull request fully migrates the project to GTK4, cleans up and fixes the old codebase, modernizes dependencies, and fixes numerous bugs and warnings

[danigm]

Big update. Of course, it's very hard to review this PR, and I'm not working on this project anymore.

If you are interested in continue with the project, and make it a bit more official, I can archive the project and add some comment in the readme to link to your fork.

[j4imefoo]

I understand :) I have just revived it cause i remember using it years ago and wanted to still have the possibility of using it. Such a nice easy-to-use program! The big update was needed just to change to gtk-4, but I have kept the use as it was intended by you. I will keep maintaining it but feel free to point it to my repository or keep yours open. I intend to upload the PKGBUILD to the Arch AUR if it's OK for you. I think I have an account there but I will have to check. Thanks again for TBO!