window: Turn gtk_window_set_screen() into gtk_window_set_display()

And have a priv->display instead of a priv->screen.

Includes turning gtk_menu_set_screen() into gtk_menu_set_display(),
because that function just forwards to its window.

demos/gtk-demo/assistant.c

@@ -170,8 +170,8 @@ do_assistant (GtkWidget *do_widget)
 
       gtk_window_set_default_size (GTK_WINDOW (assistant), -1, 300);
 
-      gtk_window_set_screen (GTK_WINDOW (assistant),
-                             gtk_widget_get_screen (do_widget));
+      gtk_window_set_display (GTK_WINDOW (assistant),
+                              gtk_widget_get_display (do_widget));
 
       create_page1 (assistant);
       create_page2 (assistant);