presentation: Add a status string to describe each output

This makes it consistent with Settings -> Displays.

https://bugzilla.gnome.org/show_bug.cgi?id=737580
presentation: Show a thumbnail for each display

Based on cc-display-panel.c from gnome-control-center.

https://bugzilla.gnome.org/show_bug.cgi?id=737580
presentation: Prevent inactive displays from being selected

eg., if one of them has been turned off from Settings -> Displays.

https://bugzilla.gnome.org/show_bug.cgi?id=737580
presentation: Frame the list and give each row a header

https://bugzilla.gnome.org/show_bug.cgi?id=737580