Bug 12644 - Add subtitles to staff client cart

Bug 12644 - Add subtitles to staff client cart

This patch adds display of subtitles to the staff client cart.

To test you should have some values mapped to subtitle in Administration
-> Keyword to MARC Mapping.

1. Apply the patch.
2. Add some records to the staff client cart which have subtitles.
3. Open the cart and onfirm that subtitles display correctly in both the
   brief and "More details" view.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>