[IMP] web: improve core Dialog API

*: base_automation, iap, web_tour

Previously, when trying to create a custom dialog, one would need to
extend the base Dialog class and configure it directly on the class,
using separate templates for the body and footer as necessary. This is
very unidiomatic owl code, and it is much more natural to simply extend
Component, and use Dialog at the root of the template, giving it slots
and props to configure its properties and contents.

This commit changes the dialog API to work as described and adapts code
that uses it.

closes odoo/odoo#89113

Enterprise: https://github.com/odoo/enterprise/pull/26462


Related: odoo/enterprise#26462
Signed-off-by: Géry Debongnie <ged@odoo.com>