Summary:
This change inits XInput extension, listens for touch events and
forwards them to our platform API. Thus touch events are forwarded on a
nested wayland session on X11.

Please note that I only tested this change on Xwayland.

Test Plan: Run nested kwin_wayland with two outputs and looked into debug console

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17369