Notify about decorations button order change

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