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).