Add paging capabilities to the List widget (Pageable mixin, Pull Request 2)

`deliteful/list/Pageable` is a mixin that add paging capabilities to the `deliteful/list/List` widget. See `deliteful/docs/list/Pageable.md` for more
information.

This pull request replaces https://github.com/ibm-js/deliteful/pull/89, based on a new branch that includes most of the changes that were requested during the review of the initial pull request.