This will allows us to support the XF86Display key present on some
laptops, directly in mutter. This is also known, in evdev, as
KEY_SWITCHVIDEOMODE.

The common usage for this key is to alternate between a few well known
multi-monitor configurations though these aren't officially
standardized. As an example, Lenovo documents it as:

"Switches the display output location between the computer display
and an external monitor."

On this patch, we're just introducing the configurations that have been
implemented in g-s-d until now, which go a bit beyond the above
description.

https://bugzilla.gnome.org/show_bug.cgi?id=781906