These settings reside in gnome-settings-daemon, but are actually accessed
all over the place. It seems to make more sense to have them here, even
though they still would be consumed by gnome-settings-daemon (x11 only,
mind you).

One of these consumers is GTK (wayland backend), possibly via the settings
portal, so it's nice to have these settings exist without gnome-settings-daemon
installed.

Fixes: #31