xfsettingsd: Add XfceGtkSettingsHelper - V2: GSettings/gtk-modules

Supersedes: !100 (closed)

Synchronization via gtk-modules requires patching GTK to work: see https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/5733.

Fixes: libxfce4ui#38 (closed)

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Related: !8 (merged), #179 (closed)