[FW][MERGE][FIX] survey, website_slides: split computed editable fields

Purpose
=======
Some computed stored editable fields are computed in the same method.

In that case, when one of the value is provided in a create / write call,
the computed method is not called and other fields computed in the same
method do not have the right value.

Task 2377119
COM odoo/odoo/pull/65772
ENT odoo/enterprise/pull/16221

closes odoo/odoo#67345

Forward-port-of: odoo/odoo#67017
Forward-port-of: odoo/odoo#65772
Related: odoo/enterprise#16894
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>