printers: Wrap GtkListBox inside a GtkBox

In order to vertically align the printers panel content by the
same standards as the other panels, we should set the GtkListBox
top and bottom margins to 32px.

This commit wraps GtkListBox inside a GtkBox to handle the Gtk+
issues discussed in Bug 773459 regarding the background of ListBox
margins.

This commit can be cleanly reverted once the issue is fixed in
Gtk+.

https://bugzilla.gnome.org/show_bug.cgi?id=786384