fstl: install desktop entry

Adds:

  - $out/share/applications/fstlapp-fstl.desktop
  - $out/share/icons/hicolor/16x16/apps/fstlapp-fstl.png
  - $out/share/icons/hicolor/22x22/apps/fstlapp-fstl.png
  - $out/share/icons/hicolor/32x32/apps/fstlapp-fstl.png
  - $out/share/icons/hicolor/48x48/apps/fstlapp-fstl.png
  - $out/share/icons/hicolor/64x64/apps/fstlapp-fstl.png
  - $out/share/icons/hicolor/128x128/apps/fstlapp-fstl.png
  - $out/share/icons/hicolor/256x256/apps/fstlapp-fstl.png

pkgs/by-name/fs/fstl/package.nix

@@ -4,15 +4,23 @@
   fetchFromGitHub,
   cmake,
   libsForQt5,
+  xdg-utils,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
   pname = "fstl";
   version = "0.11.1";
 
+  postPatch = ''
+    patchShebangs --build xdg/xdg_install.sh
+    substituteInPlace xdg/fstlapp-fstl.desktop \
+      --replace-fail 'Exec=fstl' 'Exec=${placeholder "out"}/bin/fstl'
+  '';
+
   nativeBuildInputs = [
     cmake
     libsForQt5.wrapQtAppsHook
+    xdg-utils
   ];
 
   installPhase = lib.optionalString stdenv.hostPlatform.isDarwin ''
@@ -24,6 +32,10 @@ stdenv.mkDerivation (finalAttrs: {
     runHook postInstall
   '';
 
+  postInstall = ''
+    env --chdir ../xdg XDG_DATA_HOME=$out/share ./xdg_install.sh fstl
+  '';
+
   src = fetchFromGitHub {
     owner = "fstl-app";
     repo = "fstl";