Add the system-monitor applet

[ziguana]

Closes #625

TODO:

• resolve the duplicate dependency issue
• cosmic-client-toolkit depends on registry+https://github.com/rust-lang/crates.io-index#smithay-client-toolkit@0.19.2
• cosmic-panel-config depends on git+https://github.com/Smithay/client-toolkit#smithay-client-toolkit@0.19.2
  • why git?
• buildRustPackage fails because the same version of a crate is added twice. same error as here.
• previous relevant issue: cosmic-panel-config has transitive dependency on smithay pop-os/cosmic-panel#4
• test that the applet is present and functional when built with this flake

[lilyinstarlight]

Thank you so much for this contribution! There's a few minor nits, but otherwise this looks great :)

resolve the duplicate dependency issue

This will be fixed in Nixpkgs via NixOS/nixpkgs#282798 but you can also bug upstream to add a line like this to their Cargo.toml to fix it: https://github.com/cosmic-utils/tasks/blob/406ffac6a2263cdc1006f3578b3f9be64761d6ea/Cargo.toml#L50-L51

[ziguana]

Thanks for the review! Sorry for blindly copying some useless stuff from another applet. I pushed a non-working WIP branch, as I still don't understand where to get Cargo.lock from.