(minor) gui: link to cl-pkr color picker, nice Ltk app, AppImage

gui.md

@@ -105,6 +105,7 @@ The Lisp binding is [Ltk][ltk].
 - Example applications:
  - [Fulci](https://notabug.org/cage/fulci/) - a program to organise your movie collections.
  - [Ltk small games](https://github.com/mijohnson99/ltk-small-games) - snake and tic-tac-toe.
+ - [cl-pkr](https://github.com/VitoVan/cl-pkr) - a cross-platform color picker.
  - [cl-torrents](https://github.com/vindarel/cl-torrents) - searching torrents on popular trackers. CLI, readline and a simple Tk GUI.
 - More examples:
  - [https://peterlane.netlify.app/ltk-examples/](https://peterlane.netlify.app/ltk-examples/): LTk examples for the [tkdocs](https://tkdocs.com/tutorial/index.html) tutorial.