qownnotes: add shell completion

pkgs/applications/office/qownnotes/default.nix

@@ -14,6 +14,8 @@
 , botan2
 , pkg-config
 , nixosTests
+, installShellFiles
+, xvfb-run
 }:
 
 let
@@ -34,6 +36,8 @@ stdenv.mkDerivation {
     qttools
     wrapQtAppsHook
     pkg-config
+    installShellFiles
+    xvfb-run
   ] ++ lib.optionals stdenv.isDarwin [ makeWrapper ];
 
   buildInputs = [
@@ -49,9 +53,16 @@ stdenv.mkDerivation {
     "USE_SYSTEM_BOTAN=1"
   ];
 
-  postInstall =
+  postInstall = ''
+    installShellCompletion --cmd ${appname} \
+      --bash <(xvfb-run $out/bin/${appname} --completion bash --allow-multiple-instances) \
+      --fish <(xvfb-run $out/bin/${appname} --completion fish --allow-multiple-instances)
+    installShellCompletion --cmd ${pname} \
+      --bash <(xvfb-run $out/bin/${appname} --completion bash --allow-multiple-instances) \
+      --fish <(xvfb-run $out/bin/${appname} --completion fish --allow-multiple-instances)
+  ''
   # Create a lowercase symlink for Linux
-  lib.optionalString stdenv.isLinux ''
+  + lib.optionalString stdenv.isLinux ''
     ln -s $out/bin/${appname} $out/bin/${pname}
   ''
   # Wrap application for macOS as lowercase binary