Bug 21736: Allow translating the localization widget messages

Bug 21736: Allow translating the localization widget messages

The widget used to translate the item type names to other
languages has few javascript messages that cannot be translated.
Add the mechanism to enable their translation.

Test plan:

1) Install and update a language
2) Check the po-files, there's no msgid for
   "A translation already exists for this language."
3) Apply the patch
4) Install and update a language
5) There should be a msgid for the one in 2, and
   couple other messages from localization.tt
6) Translate those messages, update the translated templates
7) Go to Administration -> Item types -> Modify item type ->
   Translate into other languages
   Add a new translation, check that the message given by the widget
   was in correct language.

Signed-off-by: Pasi Kallinen <pasi.kallinen@joensuu.fi>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>