Add new class that is a model of numbers between two values

Summary:
In QML a lot of code is built up on list models.

One thing I found tedious is a model of integers that goes N...M,
especially for use in Tumblers and Repeaters.

Test Plan: Unit test

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: broulik, markg, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13358