Add gtk_native_dialog_destroy()

Its very easy to get extra references to the NativeDialog so that
when you release your last reference any visible dialog is not
hidden. We handle this by adding a destroy method similar to how
you destroy regular toplevels.

demos/gtk-demo/application.c

@@ -113,6 +113,7 @@ open_response_cb (GtkNativeDialog *dialog,
         }
     }
 
+  gtk_native_dialog_destroy (GTK_NATIVE_DIALOG (native));
   g_object_unref (native);
 }