Update gtk-rs dependencies and fix linter errors

These changes update the gtk-rs dependencies, in particular setting the gtk4 version requirement to 0.5.3 to get the fix for this gtk4-rs issue. Additionally this addresses some clippy warnings and fixes the RUST_SRC_PATH variable in the nix shell which was preventing the rust-analyzer VSCode plugin from running.

This should fix the panic seen in #174 (closed). This merge request includes the changes from and supersedes !189 (closed).