Backport of PR #86157.

Motivation for this change

New releases came out. Was hoping the update bot would pick up on them like before but it didn't, prolly due to the project moving to a different location.

Things done

Src changed to the new location of the project. Configuration updated based on the current build settings requirements. A GTK3 option has been added as it's now officially sanctioned to be used with the official branding. GTK2 still seems to be the default, but I briefly tested a x86_64 GTK3 build and it seemed to work fine. i686 builds is entirely untested on my end.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
  531405504 (28.8.1, haven't updated in awhile) -> 474916144
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.