Bug 10576: make strings in additem.js accessible to translation

Bug 10576: make strings in additem.js accessible to translation

There are a couple of untranslatable strings in additem.js. This patch
moves the strings out of the script and into the include file which has
been created for this purpose.

To test, apply the patch and test the process for adding an item to an
existing or new basket (with AcqCreateItem set to "when placing an
order."

The add item form should be correctly labeled "Add item." After adding
an item, click to edit it again. The form should now be labeled "Update
item."

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works nicely and passes all tests.
Made sure strings can be translated testing with German templates.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>