[nightcolor] Use a dedicated class for detection of system clock changes

Summary:
The ClockSkewNotifier provides a convenient way for monitoring system
clock changes. One of the key ideas was to hide platform details from
users of the class. This allows us to add a QTimer fallback path for
operating systems that don't provide anything to detect system clock
changes.

In long term, I would like to move the new class to Frameworks.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D25962