meta-launcher: Explicitly look up object path for seat

If mutter is running as a systemd user service, then we cannot use the
magic "self" session for the ID lookup. For now we need to lookup the ID
explicitly. Eventually we can change to use the magic "auto" paths for
both the session and seat, but that will require systemd v243.

See also systemd/systemd#12424 (comment)

https://gitlab.gnome.org/GNOME/mutter/merge_requests/571

src/backends/native/meta-launcher.c

@@ -251,13 +251,15 @@ get_session_proxy (GCancellable *cancellable,
 }
 
 static Login1Seat *
-get_seat_proxy (GCancellable *cancellable,
+get_seat_proxy (gchar        *seat_id,
+                GCancellable *cancellable,
                 GError      **error)
 {
+  g_autofree char *seat_proxy_path = get_escaped_dbus_path ("/org/freedesktop/login1/seat", seat_id);
   Login1Seat *seat = login1_seat_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                                          G_DBUS_PROXY_FLAGS_DO_NOT_AUTO_START,
                                                          "org.freedesktop.login1",
-                                                         "/org/freedesktop/login1/seat/self",
+                                                         seat_proxy_path,
                                                          cancellable, error);
   if (!seat)
     g_prefix_error(error, "Could not get seat proxy: ");
@@ -493,6 +495,7 @@ meta_launcher_new (GError **error)
   g_autoptr (Login1Session) session_proxy = NULL;
   g_autoptr (Login1Seat) seat_proxy = NULL;
   g_autofree char *seat_id = NULL;
+  g_autofree char *seat_proxy_path = NULL;
   gboolean have_control = FALSE;
 
   session_proxy = get_session_proxy (NULL, error);
@@ -511,7 +514,7 @@ meta_launcher_new (GError **error)
   if (!seat_id)
     goto fail;
 
-  seat_proxy = get_seat_proxy (NULL, error);
+  seat_proxy = get_seat_proxy (seat_id, NULL, error);
   if (!seat_proxy)
     goto fail;