Summary:
This ensures that the X11 specific events are only read on X11 but not
on Wayland. Also moves quite some X11 specific code out of TabBox.
Test Plan: Compile checked, currently on Wayland, so no way to test.
Reviewers: #kwin, #plasma
Subscribers: plasma-devel, kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D7395
