Approved employee leave can now create timesheet entries automatically, using the employee's working calendar to calculate time off. Companies can configure the default project and task, and leave types can decide whether timesheets should be generated, reducing manual tracking and improving reporting accuracy.

Sub of https://github.com/odoo/odoo/pull/15526/commits

Task: https://www.odoo.com/web#id=29568&view_type=form&model=project.task&action=333&active_id=965&menu_id=4720

Updates to custom models and fields are now handled more safely and consistently, reducing the chance of partial changes that can leave databases in an inconsistent state. The registry and cache refresh process was streamlined so changes are applied transactionally at the end of a request, making customizations easier to extend and maintain.

Refactor the code, and improve it in the following ways:
- remove deprecated class `RegistryManager`;
- simplify APIs of registry setup by removing parameter `partial`;
- delegate addition of custom models/fields to models `ir.model` and `ir.model.fields`, which makes it easily extensible;
- always perform registry/cache signaling at the end of request, and make updates on custom models/fields fully transactional.