wayfire: allow path in settings

[einetuer]

Description

Allow a store path to be specified in wayfire's settings. Not strictly necessary, but having to call toString when setting wallpaper, cubemap background etc gets pretty annoying and feels unnecessary.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt-rfc-style deadnix keep-sorted --run treefmt.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.

• 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.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

[trueNAHO]

LGTM.