gtk-layer-shell: install demo

pkgs/development/libraries/gtk-layer-shell/default.nix

···
   version = "0.7.0";
 
   outputs = [ "out" "dev" "devdoc" ];
+  outputBin = "devdoc"; # for demo
 
   src = fetchFromGitHub {
     owner = "wmww";
···
 
   mesonFlags = [
     "-Ddocs=true"
+    "-Dexamples=true"
   ];
 
   meta = with lib; {