Bug 14100: Generic solution for language overlay - Item types

Bug 14100: Generic solution for language overlay - Item types

Test plan:
1/ update the Schema (misc/devel/update_dbix_class_files.pl)
2/ Translate templates for some languages (es-DE, de-DE for instance)
3/ Enable them in the pref (search for 'lang') for the staff interface
4/ Go on the item type admin page (admin/itemtypes.pl)
5/ Edit one
6/ Click on the 'translate for other languages' link
7/ You are now on the interface to translate the item type's description
in the languages you want. So translate some :)
8/ Go back on the item type list view (admin/itemtypes.pl)
9/ You should see the original description (non translated)
10/ Switch the language
11/ You should see the translated description in the correct language.
If the description is non translated, the original description is
displayed.

12/ On the different page where the item type is displayed, confirm that
the translated description appears.

Think further / Todo:
1/ Update all occurrences of the item type's description (DONE)
2/ Implement for authorised values
3/ Implement for syspref value (at least textarea)
4/ Implement for branch names
5/ Centralize all the translation on a single page in the admin area
...
N/ Implement a webservice to centralize all the translations and give
the ability to sync the item types/authorised values description with
the rest of the world (push and pull).

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>