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
