WIP: Lock screen when the lid is closed without external monitors, even if "suspend on lid close" is disabled (Closes: #919914)

Should fix https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=919914, according to @jbicha on https://gitlab.gnome.org/GNOME/gnome-settings-daemon/merge_requests/84.

WIP as I haven't tested this yet.