[FIX] project: make default task order more natural

As of v11, project tasks and project issues have been merged.

Issues used to be ordered by:

        priority desc, create_date desc

whereas Tasks had the following historical ordering:

        priority desc, sequence, date_start, name, id

(where `date_start` corresponds to the creation date of the task, or the
time where task work began)

After the merge, the Task order was kept, but it became difficult to
understand for users, especially for converted issues.

This commit modifies the default order to a more obvious common order:

       priority desc, sequence, id desc

With the following goals in mind:
- the ordering does not change without obvious actions of the user
  (starring a record moves it at the top, manually reordering records
   reorders them within the same "stars category), and creating new
   records adds them at the top, where they are visible
- the ordering is easy to grasp, based on stars and chronological
  order, while still allowing manual reordering when really necessary
  (by default all records will have the default sequence of 10)
- sorting by "id desc" is similar to the previous "create_date desc"
  but it can be sorted more efficiently by the database

One could argue about sorting on chronological or reverse chronological
order. Like blogs and timelines, everyone has their preferences.
The latter option was selected because our UI for quickly adding records
is at the top of the kanban view, so it is more natural to see new
records at the top, instead of having them disappear at the bottom
immediately.

Hopefully this will not disrupt user workflows too much, and will remain
intuitive enough.

addons/project/models/project.py

@@ -367,7 +367,7 @@ class Task(models.Model):
     _date_name = "date_start"
     _inherit = ['mail.thread', 'mail.activity.mixin', 'portal.mixin']
     _mail_post_access = 'read'
-    _order = "priority desc, sequence, date_start, name, id"
+    _order = "priority desc, sequence, id desc"
 
     def _get_default_partner(self):
         if 'default_project_id' in self.env.context: