kdeconnect: trigger indicator service after generic tray.target (#6711)

Each of `polybar.service`, `stalonetray.service` and `taffybar.service`
contains `WantedBy=tray.target`. So we can use a single `tray.target` in
`kdeconnect-indicator.service`'s `After=`.

This also makes the applet service tray implementation agnostic, so long
as it integrates with `tray.target`.

modules/services/kdeconnect.nix

@@ -68,12 +68,7 @@ in {
       systemd.user.services.kdeconnect-indicator = {
         Unit = {
           Description = "kdeconnect-indicator";
-          After = [
-            "graphical-session.target"
-            "polybar.service"
-            "taffybar.service"
-            "stalonetray.service"
-          ];
+          After = [ "graphical-session.target" "tray.target" ];
           PartOf = [ "graphical-session.target" ];
           Requires = [ "tray.target" ];
         };