Bug 9456 - Add callnumber column to the cart

This patch adds call-number sorting to the staff client cart. Sortining is enabled
on the 'Items' column which uses a custom sorting routine to sort the callnumbers.
If the sort is ascending, each column will be sorted by the highest ordered
callnumber for that row. Likewise, if the sort is descending the table will be
sorted by the lowest valued callnumber for that row.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>