ir.ui.menu was recently changed to use _parent_store,
which precludes using ondelete=set null for the parent_id
column. We nevertheless need to be certain that menus
can always be deleted but *never* cascade-deleted,
due the possible presence of user-created menus.
Overriding menu.unlink() is therefore necessary,
and care must be taken to bypass the default menu
visibility (using the `ir.ui.menu.full_list` context
flag while doing so)
bzr revid: odo@openerp.com-20121213145821-u2ipdvffu00rsgdg