Summary:
This allows GtkConfig daemon to properly sync decorations buttons order for GTK CSD applications.
For 5.18.3
Test Plan:
1. Launch `kcmshell5 kwindecoration` and any GTK CSD app alongside
2. Change order of the buttons.
3. Window button order should be changed in GTK CSD app immediately (if `xsettingsd` is installed).
Subscribers: kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D27904