Bug 13170 Remove of prog theme broke the OPAC's "view plain" option for MARC details

Bug 13170 Remove of prog theme broke the OPAC's "view plain" option for MARC details

In the OPAC if you view the MARC details for a title (and have
OPACXSLTDetailsDisplay enabled) there is a "view plain" link which displays the
output of opac-showmarc.pl. This is broken in master: fixed by this patch.

Test plan:

(1) Set OPACXSLTDetailsDisplay to default
(2) Do a search on OPAC, then display a specific biblio record
(3) Click on MARC view tab. Then click on 'view plain' link. Nothing is
    displayed.
(4) Apply the patch. And refresh the MARC detail page.
(5) Click on 'view plain' link. Check that a plain text MARC record is
    displayed.

Signed-off-by: Chris <chris@bigballofwax.co.nz>
Note: This makes a small change to C4::Templates::themelanguage so that
it works with .xsl files too (They live in the xslt dir)

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works as described, passes tests and QA script.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>