Added GridSlaves and fixed GridForget

Hi, (I could be wrong and not using GridForget correctly.) While converting my old Python ToDo app I needed the 'grid slaves' and 'grid forget' directives. I made Gridslave however I needed to fix GridForget to get it working. It's not the Window you need to pass but the widgets. tk.Row() and tk.Column() are a valid option for GridSlave().

func (me *App) deleteTask(r int) {

	fmt.Println(me.inner_frame.GridSlaves())
	tk.GridForget(me.inner_frame.GridSlaves(tk.Row(r)))
}

GridSlave:

func (w *Window) GridSlaves(option ...Opt) []string {
	if option == nil {
		return parseList(evalErr(fmt.Sprintf("grid slaves %v", w)))

	} 
	return parseList(evalErr(fmt.Sprintf("grid slaves %v %s", w, collect(option...))))
	
}

GridForget:

func GridForget(widget []string) {
	if len(widget) == 0 {
		return
	}
	evalErr(fmt.Sprintf("grid forget %s", strings.Join(widget, " ")))
}