[FIX] ir.translation: backport of trunk fix to allow editing master english source terms

Original trunk commits rev-ids:
 - 4855 qdp-launchpad@openerp.com-20130416091501-sgm3gdm80oxajahz
 - 4856 qdp-launchpad@openerp.com-20130417121718-jfwi2f3gn0tum9xa
 - 4857 qdp-launchpad@openerp.com-20130418114627-unmfdu6010hyc8xx