Remove pollinglistwalker

Removes the PollingListWalker class and raises a ListBoxError when a ListBox is created without passing an object descending from ListWalker class. Despite its name, PollingListWalker is/was *not* inheriting from the ListWalker class.

Also changes unit tests and example programs to always use a ListWalker object everywhere a ListBox is created.

Fixes #37