Added support to automatically bind events when using Builder.

type Test struct {
	patata int
}

func (*Test) On_button1_clicked(button *gtk.Button) {
// clicked
}

.....


	if builder, err = gtk.BuilderNew(); err != nil {
		log.Fatalf("Error: %s", err.Error())
	}

	builder.AddFromFile("gui.builder")
	builder.BindCallbacks(builder, &Test{})



 Changes to be committed:
	new file:   glib/glib_extra.go
	modified:   gtk/gtk.go
	new file:   gtk/gtkbuilder_bridge.go
	new file:   gtk/gtkbuilder_cgo.go