Revert "ComboBox: Don’t redundantly show() scrolled_window"

This reverts commit 03ead0c824.

That was a real brainfart, which broke showing list-mode CBs on the 2nd
and subsequent popups.

gtk/gtkcombobox.c

@@ -2090,6 +2090,7 @@ gtk_combo_box_real_popup (GtkComboBox *combo_box)
                                     GTK_WINDOW (toplevel));
     }
 
+  gtk_widget_show_all (priv->scrolled_window);
   gtk_combo_box_list_position (combo_box, &x, &y, &width, &height);
   
   gtk_widget_set_size_request (priv->popup_window, width, height);