nixos/cinnamon: add missing xapp icons

Mostly all mint app looks for xapp-* icons, while some icon themes do provide them,
mint-y-* does not. We are just going the laziest way here to install xapp globally
to get those icons.

nixos/modules/services/x11/desktop-managers/cinnamon.nix

@@ -180,6 +180,7 @@ in
         mint-themes
         mint-x-icons
         mint-y-icons
+        xapp # provides some xapp-* icons
       ] config.environment.cinnamon.excludePackages);
 
       xdg.mime.enable = true;