Summary:
Window decorations button order was applied only for window headers that was controlled by KWin, but not for GTK applications with CSD.  Now it is no longer true - button order in CSD applications are in sync with the one used by KWin.

Only Close, Maximize, Minimize and Icon buttons are synchronized, because GTK supports only them.

Depends on D25695

Test Plan:
# Open two windows alongside each other: window decorations button order settings and any gtk3 app with CSD (for example, Lutris)
# Restart kded5
# Apply any WD button order, apply settings
# The app should change its buttons order in headerbar (if xsettingsd is not installed, on X11 only after restart)
{F7794441}

Reviewers: #vdg, cblack, #plasma, apol

Reviewed By: apol

Subscribers: apol, GB_2, broulik, plasma-devel

Tags: #plasma

Maniphest Tasks: T10611

Differential Revision: https://phabricator.kde.org/D25670