shell: Swap Display and Printers positions

So that whenever the user opens the "Devices" section,
the Display panel is the first panel to dislpay. This
is because the user most likely always have a display,
while they might not always have a printer available.

https://bugzilla.gnome.org/show_bug.cgi?id=786606

shell/cc-panel-list.c

@@ -289,10 +289,10 @@ static const gchar * const panel_order[] = {
   "network",
 
   /* Devices page */
-  "printers",
+  "display",
   "keyboard",
   "mouse",
-  "display",
+  "printers",
   "removable-media",
   "wacom",
   "color",