dunst: make icon_path extensible

[ncfavier]

Description

This makes settings.global.icon_path a separatedString ":" so that the user can add their own paths.

I need this because the current icon_path setting is quite flaky (but I don't think there's a better way apart from implementing the icon theme specification, or waiting until dunst does).

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

[berbiche]

Thanks for the contribution!

[berbiche]

Tested locally 👍

[ncfavier]

Thanks!