Provide an implementation for the tablet interface

Summary:
Implements the necessary classes to have proper support for the tablet
and pen.
Doesn't implement yet the ring/slider/totem bits.

Test Plan:
Used it with Plasma and GTK clients, together with the kwin patch I'll
submit soon.
https://youtu.be/GGx0TlNJlzs

Also I added a test.

Reviewers: KWin, Frameworks

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26858