gtk3: restore built-in icons from 3.12

http://lists.nongnu.org/archive/html/mingw-cross-env-list/2017-11/msg00000.html
closes #1968

src/gtk3-test.c

@@ -7,14 +7,22 @@
 static void activate(GtkApplication *app)
 {
   GtkWidget *window;
-  GtkWidget *button;
+  GtkWidget *box;
+  GtkWidget *spinbutton;
+  GtkWidget *comboboxtext;
 
   window = gtk_application_window_new(app);
-  button = gtk_button_new_with_label("Hello World");
-
-  g_signal_connect_swapped(button, "clicked",
-               G_CALLBACK(gtk_widget_destroy), window);
-  gtk_container_add(GTK_CONTAINER(window), button);
+  box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0);
+  spinbutton = gtk_spin_button_new_with_range(0, 10, 1);
+  comboboxtext = gtk_combo_box_text_new();
+  gtk_combo_box_text_append_text(GTK_COMBO_BOX_TEXT(comboboxtext), "One");
+  gtk_combo_box_text_append_text(GTK_COMBO_BOX_TEXT(comboboxtext), "Two");
+  gtk_combo_box_text_append_text(GTK_COMBO_BOX_TEXT(comboboxtext), "Three");
+  gtk_combo_box_set_active(GTK_COMBO_BOX(comboboxtext), 0);
+
+  gtk_container_add(GTK_CONTAINER(window), box);
+  gtk_box_pack_start(GTK_BOX(box), spinbutton, TRUE, TRUE, 0);
+  gtk_box_pack_start(GTK_BOX(box), comboboxtext, TRUE, TRUE, 0);
 
   gtk_widget_show_all(window);
 }