gdkdisplay: Deprecate gdk_display_notify_startup_complete()

We have various layers where we store the startup ID for a request,
since this API does not have a GdkToplevel that we can refer about
for the Wayland platform, this is the most obvious candidate to
start untangling these various layers.

Deprecate this call, it is already unused in the gtk/ side.

gdk/gdkdisplay.c

@@ -1148,6 +1148,8 @@ _gdk_display_get_next_serial (GdkDisplay *display)
  * with custom startup-notification identifier unless
  * [method@Gtk.Window.set_auto_startup_notification]
  * is called to disable that feature.
+ *
+ * Deprecated: 4.10. Using gdk_toplevel_set_startup_id() is sufficient.
  */
 void
 gdk_display_notify_startup_complete (GdkDisplay  *display,

gdk/gdkdisplay.h

@@ -83,7 +83,7 @@ GdkClipboard *          gdk_display_get_clipboard               (GdkDisplay
 GDK_AVAILABLE_IN_ALL
 GdkClipboard *          gdk_display_get_primary_clipboard       (GdkDisplay     *display);
 
-GDK_AVAILABLE_IN_ALL
+GDK_DEPRECATED_IN_4_10_FOR(gdk_toplevel_set_startup_id)
 void     gdk_display_notify_startup_complete   (GdkDisplay    *display,
                                                 const char    *startup_id);
 GDK_AVAILABLE_IN_ALL

gtk/gtkwindow.c

@@ -5896,7 +5896,7 @@ _gtk_window_set_is_active (GtkWindow *window,
  * Sets whether the window should request startup notification.
  *
  * By default, after showing the first `GtkWindow`, GTK calls
- * [method@Gdk.Display.notify_startup_complete]. Call this function
+ * [method@Gdk.Toplevel.set_startup_id]. Call this function
  * to disable the automatic startup notification. You might do this
  * if your first window is a splash screen, and you want to delay
  * notification until after your real main window has been shown,