services.plasma5: Update start menu with an activationScript

To update the plasma start menu `kbuildsyscoca5` needs to be executed.
There are several people complaining about missing applications in their
plasma start menu.
This patch adds a activationScript for plasma, that runs
`kbuildsyscoca5` for each user that has `isNormalUser` == `true`.

nixos/modules/services/x11/desktop-managers/plasma5.nix

@@ -221,6 +221,11 @@ in
       security.pam.services.sddm.enableKwallet = true;
       security.pam.services.slim.enableKwallet = true;
 
+      # Update the start menu for each user that has `isNormalUser` set.
+      system.activationScripts.plasmaSetup = stringAfter [ "users" "groups" ]
+        (concatStringsSep "\n"
+          (mapAttrsToList (name: value: "${pkgs.su}/bin/su ${name} -c kbuildsycoca5")
+            (filterAttrs (n: v: v.isNormalUser) config.users.users)));
     })
   ];