Notify about window decorations buttons order changes

Summary:
This makes it possible to detect these changes via KConfigWatcher.

This is needed for D25670

Reviewers: KWin, ngraham, davidedmundson

Reviewed By: KWin, ngraham, davidedmundson

Subscribers: ngraham, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D25695