[MERGE] account_voucher, product, stock: use selection widgets for magic context...

Many2one fields resolve to strings when used
as `self` within the @context or @filter_domain
attributes of a search field, because they
must be able to support partial matches,
e.g. search for "Sto" to match "Stock".
In many case the code was not prepared to accept
string for the magic context values, so the
magic fields now caused crashes.

The widget="selection" attribute was dropped
during the search view cleanup for 7.0,
without noticing this unfortunate side-effect,
at revision:
  addons 7.0 revno 7142 revid:qdp-launchpad@openerp.com-20120731150358-jqd3eoz06imzlx01

lp bug: https://launchpad.net/bugs/1192484 fixed

bzr revid: odo@openerp.com-20131004122257-5cx5sf3nm8qi5auc