Always use the default screen

GDK doesn't support multiple screens, so effectively we don't either.
Lets stop pretending we do.

This fixes a few -Wdeprecated warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=769070

src/core/startup-notification.c

@@ -675,7 +675,7 @@ meta_startup_notification_constructed (GObject *object)
                                    sn_error_trap_pop);
   sn->sn_context =
     sn_monitor_context_new (sn->sn_display,
-                            sn->display->screen->number,
+                            meta_ui_get_screen_number (),
                             meta_startup_notification_sn_event,
                             sn,
                             NULL);