values for next activities are currently prepared inside the `mail_activity._action_done()` method which makes inheritance very difficult. This commit delegates the value computation to a sub-method and thereby improves the extensibility of the module. required to support changes to enterprise documents odoo/enterprise#20769 Task-2627837 closes odoo/odoo#77004 X-original-commit: a26e6e954c5e6c773f37e9239138a7b94ded821f Related: odoo/enterprise#21079 Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>