bug 7494: optional checkout-time fallback search for a book

When issuing a book, some libraries want to issue by title or other details.
This patch adds a systempreference and code that allows it.

To test:
1) scan a patron card or enter a surname to start checking out;
2) enter title or other keywords;
3) the circulation screen should display a warning allowing to choose between copies.

Signed-off-by: MJ Ray <mjr@phonecoop.coop>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
bug 7494: (follow-up) tweak description of itemBarcodeFallbackSearch syspref

Thanks to MJ Ray for helping with the wordsmithing on #koha.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
bug 7494: DBRev 3.13.00.015'

Signed-off-by: Galen Charlton <gmc@esilibrary.com>