Implement the tablet wayland protocol in kwin

Summary:
Uses the tablet classes introduced in kwayland.
Depends on D26858

Test Plan:
Scratched my tablet with a magic stick and it did things depending on the pressure.
https://youtu.be/GGx0TlNJlzs

Reviewers: #kwin, #plasma, zzag, davidedmundson

Reviewed By: #kwin, #plasma, zzag, davidedmundson

Subscribers: davidedmundson, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26859