Bug 12973: Allow passing code to XSLT_Handler instead of filename

Bug 12973: Allow passing code to XSLT_Handler instead of filename

This patch adds the option to pass a code string to the XSLT handler object
instead of passing a filename.
This is achieved by a change to parameter hashref. The old style parameters
are also allowed.
Note that this patch should not change behavior but will be used in future
development: see bug 12758 or bug 12754.

Test plan:
Verify that behavior did not change in the following steps.
[1] Edit OPACXSLTResultsDisplay in order to use XSLT.
[2] Check search results page in OPAC.
[3] Edit OPACXSLTDetailsDisplay in order to use XSLT.
[4] Check detail page in OPAC.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>