RATIONALE
Remove _onchange_template_id / defaults usage for composer fields. It
lacks control and make value computation hard to understand and predict.
PURPOSE
Split _onchange_template_id + default_get + get_record_data usage into editable
computed stored fields. Each field should have its own method with its triggers
(template, composition mode, ...).
SPECIFICATIONS
Improve composer to correctly compute field ``email_add_signature`` in
compute method.
When having a template, consider it defines completely body and do not add
signature. Without template, add signature by default in comment due to post
processing for notification emails. Mailing mode does not handle signature.
Followup of odoo/odoo#107356
Task-2088884 (Mail: Use editable computed stored fields in composer)
closesodoo/odoo#113522
Related: odoo/enterprise#37478
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>