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