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, zzag, davidedmundson

Reviewed By: #kwin, zzag, davidedmundson

Subscribers: davidedmundson, zzag, kde-frameworks-devel

Tags: #frameworks

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