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:

1. Change settings backend from rc files to xfconf. Changes done via the xfce4-settings-editor are not applied.
2. Refactor settings to based on a GObject config (like pulseaudio-plugin and notes-plugin).