Initial support for tablets on Wayland

Summary:
This includes support for them on libinput and turns it into fake
pointer actions.
This doesn't implement zwp_tablet, this will have to happen in an
iteration later.

Test Plan:
Been playing around with it, see video.
https://www.youtube.com/watch?v=GF1WbO8FVvU

Reviewers: #plasma, #kwin, romangg

Reviewed By: #plasma, #kwin, romangg

Subscribers: zzag, davidedmundson, romangg, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25663