Port to xfconf
Was part of https://wiki.xfce.org/releng/4.18/roadmap, now postponed to 4.20.
This can be done in two steps:
- Change settings backend from rc files to xfconf. Changes done via the xfce4-settings-editor are not applied.
- Refactor settings to based on a GObject config (like pulseaudio-plugin and notes-plugin).