Bug 13564: Items search: rename Stock number to Inventory number

Bug 13564: Items search: rename Stock number to Inventory number

To test:
- Go to Advanced search
- Click on link 'go to item search'
- Check the search options in the second section for "Stock number"
- Do a search and note the column name appears as "Stock number"
- Apply patch
- Try the search again and check that "Inventory number" shows

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works as described, thx!

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