[tabBox] Move X11 specific event filtering into a dedicated event filter

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