Callbacks on Item update in List Box

This PR is meant to address #820. I was thinking to use `after_do` to manage the callback here. But I'm struggling because for some reason `list_box.gui` is returning nil. I checked `list_box` and it is an instance of the ListBox class as expected, but it's `@gui` doesn't seem to be setup when the callback goes through . . .

Am I using after_do correctly?