gdk/wayland: Set a startup notification

And notify the shell about it. This is done through the
gtk_shell1.notify_launch request added in gtk-shell v3. All the plumbing
on the way to the activated application is already in place to transfer
the startup ID, so the other side just has to reply with
gtk_surface1.request_focus.

Closes: https://gitlab.gnome.org/GNOME/gtk/issues/624

gdk/wayland/gdkapplaunchcontext-wayland.c

@@ -35,7 +35,20 @@ gdk_wayland_app_launch_context_get_startup_notify_id (GAppLaunchContext *context
                                                       GAppInfo          *info,
                                                       GList             *files)
 {
-  return NULL;
+  GdkWaylandDisplay *display;
+  gchar *id = NULL;
+
+  g_object_get (context, "display", &display, NULL);
+
+  if (display->gtk_shell_version >= 3)
+    {
+      id = g_uuid_string_random ();
+      gtk_shell1_notify_launch (display->gtk_shell, id);
+    }
+
+  g_object_unref (display);
+
+  return id;
 }
 
 static void