Sync display settings to Greeter

[lenemter]

Required for elementary/greeter#693

[tintou]

It should be possible to define a system-wide setting by copying the monitor file to one of the GLib.Environment.get_system_config_dirs () directly.

[lenemter]

@tintou I tried to do that but it requires root to access the file "/etc/xdg/monitors.xml". Seems like there is no easy way to access it using polkit. There is a good stackoverflow answer on this here but is this really what we want to do?