This implements most of the new Display panel re-design by Allan
Day.

Left out for now is brightness setting which still is in the
Power panel because the plumbing layers don't yet associate backlights
with outputs.

Also left out is the presentation mode and trimming of options due to
hardware constraints since we don't have a mutter implementation for
those yet.

https://bugzilla.gnome.org/show_bug.cgi?id=785949