Create a button to add an item to the data list

additem.go

add := widget.NewButton("Add", func() {
	w := myApp.NewWindow("Add Data")

	itemName := widget.NewEntry()

	addData := widget.NewButton("Add", func() {
		data.Append(itemName.Text)
		w.Close()
	})

	cancel := widget.NewButton("Cancel", func() {
		w.Close()
	})

	w.SetContent(container.New(layout.NewVBoxLayout(), itemName, addData, cancel))
	w.Resize(fyne.NewSize(400, 200))
	w.CenterOnScreen()
	w.Show()

})