Bug 12999: Untranslatable string on enhanced_content.pref

Bug 12999: Untranslatable string on enhanced_content.pref

This patch modifies LangInstaller.pm to catch strings
on syspref files that have no associated syspref

To test:
1) Update strings for your preffered language
2) Check that the string "you can only choose one source"
is not present on xx-YY-pref.po file

3) Apply the patch
4) Update strings again
5) Check the strig now appears [ and another one :) ]

Doing a diff before and after will show
 # Enhanced Content > All
 msgid "enhanced_content.pref## <strong>NOTE:</strong> you can only
 choose one source of cover images from below, otherwise Koha will show
 the images from all sources selected."
 msgstr ""

 # Local Use
 msgid "local_use.pref## Nothing defined yet."
 msgstr ""

6) Translate the string
7) Install your language, check Administration > System preferences >
   Enhanced content > All
   (on your languge) and check the string is now translated

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>