Summary:
The KWindowSystem call which we used doesn't work on Wayland as it's only
implemented in the xcb variant and cannot be made available for Wayland
in an easy way as it is still XLib based.

This change turns the optional XCB-ICCCM dependency in a required one
and thus can use the functionality provided by said library to implement
what KWindowSystem provided.

BUG: 382789

Test Plan: New test case which failed with old code

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7046