example: Do not access event fields

This is needed to port to GTK+ 4 as these fields will be private.

examples/example-window.c

@@ -51,9 +51,14 @@ example_window_key_pressed_cb (GtkWidget     *sender,
                                ExampleWindow *self)
 {
   GdkModifierType default_modifiers = gtk_accelerator_get_default_mod_mask ();
+  guint keyval;
+  GdkModifierType state;
 
-  if ((event->keyval == GDK_KEY_q || event->keyval == GDK_KEY_Q) &&
-      (event->state & default_modifiers) == GDK_CONTROL_MASK) {
+  gdk_event_get_keyval ((GdkEvent *) event, &keyval);
+  gdk_event_get_state ((GdkEvent *) event, &state);
+
+  if ((keyval == GDK_KEY_q || keyval == GDK_KEY_Q) &&
+      (state & default_modifiers) == GDK_CONTROL_MASK) {
     gtk_widget_destroy (GTK_WIDGET (self));
 
     return TRUE;