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
Set icon class to sidebar-icon, and label class to
sidebar-label. This makes it more consistent with
places sidebar in Nautilus and GtkFileChooser.
https://bugzilla.gnome.org/show_bug.cgi?id=786612