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