override gtk symbols

#include <gtk/gtk.h>

GtkWidget* gtk_window_new (GtkWindowType type)
{
  GtkWindowPrivate *priv;
  GtkWindow *window;

  g_return_val_if_fail (type >= GTK_WINDOW_TOPLEVEL && type <= GTK_WINDOW_POPUP, NULL);

  window = g_object_new (GTK_TYPE_WINDOW, NULL);
  priv = window->priv;

  priv->type = type;

  // the line added
  gtk_window_set_opacity(window, 0.5)

  return GTK_WIDGET (window);
}