Bug 24740: Use biblio title rather than biblio number in OPAC search result cover...

Bug 24740: Use biblio title rather than biblio number in OPAC search result cover images tooltips

In OPAC biblionum doesn't mean much to patrons, use biblio title field if not empty.

Test plan:
OPAC search
1) Apply the patch
2) For each cover images providers available to tester in:
/cgi-bin/koha/admin/preferences.pl?tab=&op=search&searchfield=Cover
OPACLocalCoverImages, AdlibrisEnabled, OPACAmazonCoverImages, GoogleJackets, Coce provider.
3) Enable the provider.
4) Search a Library Catalog for a book with a cover image.
6) test mouse over cover image display biblio title.

OPAC Shelves
1) Enable OPAC virtualshelves preference
2) Create a list.
3) For each covers providers available.
4) Search the List
5) test mouse over cover image display biblio title.

Signed-off-by: Stina Hallin <stina.hallin@ub.lu.se>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>