Bug 15485: Introduce new sysprefs for lists XSLTs

Bug 15485: Introduce new sysprefs for lists XSLTs

This patch introduces two new sysprefs, allowing the users to
set custom XSLTs for lists display. This is done almost the usual
way.

If none is specified (value='') it defaults to the current behaviour:
using the *Results.xsl file for the corresponding interface.

Sponsored-by: Carnegie Stout Library
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Deb Stephenson <DStephen@dubuque.lib.ia.us>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>