gtklock-userinfo-module: Add custom Icon path

[vunnyso]

Description of changes

With this patch user can set custom image when the system is locked for identification purpose.

Checklist for things done

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
• PR linked to architecture documentation and requirement(s) (ticket id)
• Test procedure described (or includes tests). Select one or more:
  • Tested on Lenovo X1 x86_64
  • Tested on Jetson Orin NX or AGX aarch64
  • Tested on Polarfire riscv64
• Author has run make-checks and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status
• Change requires full re-installation
• Change can be updated with nixos-rebuild ... switch

Instructions for Testing

• List all targets that this applies to:
• Is this a new feature
  • List the test steps to verify:
• If it is an improvement how does it impact existing functionality?
  User Image can be set when system is locked

Expected output when image is kept under

[ghaf@gui-vm:~]$ ls /home/ghaf/Pictures/.face (any jpg file is fine, recommended to use circular image)

[kajusnau]

Great work!

It would be nice to have a more convenient way to update the user profile picture in the future, maybe through the control panel.

[milva-unikie]

Tested on Lenovo-X1 (nixos-rebuild switch)

• Setting an image works
• Non-circular images might get cropped a bit weirdly but they also work
• Image is preserved between boots
• Test-automation passes