[FIX] stock: forbid adding new lines when processing picking

Source and destination locations are required and not displayed in the form view.
Adding new items when recieving a picking can not be easily guessed as we can put different locations for each line, using default locations may not be the expected result.
Instead should modify the original picking or create new one.
Fixes #2074, opw 612768
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32