[IMP] web: SearchPanel: default filter values in context

Before this rev., it was possible to specify default values for
categories (fields with attribute select="one") in the context
(e.g. searchpanel_default_fieldname: 5). It is now possible to
specify default values for filters (select="multi") as well
(e.g. searchpanel_default_fieldname: [1,2]).

closes odoo/odoo#31107