[IMP] calendar: enable change recurrence start/stop

When editing a recurring event clicking `Update only this instance`
allow us to detach this event from the recurring sequence, but without
clicking it there are instances where the event will still be detached
maybe unexpectedly.

Clicking on it will create an event which shadow the recurring one that
being done thanks to:

- recurring_id: the id of the recurring event creating this event
- recurring_id_date: start of the virtual event this new event shadows

Before this commit: when the start or end date of an recurring event is
changed, all modification are not applied on the recurring events but
only the current virtual event occurrence is detached and modified.

After this commit:

- when the end date of a recurring event is modified, it is modified for
  all the instance (not detached) of the recurring event,

- when the start date of a recurring event is modified, it is modified
  all the instance (not detached) of the reccuring event and if:

  * one occurrence of the recurring event is still on a (not-moved)
    detached event of this recurrence, this virtual event will be
    shadowed.

  * a detached event has now no occurrence of the recurring event on its
    day, it will still be left untouched but will now not shadow an
    event anymore.

eg.

we start with recurring event A on 4, 5, 6 => we see [4: A, 5: A, 6: A]

-> detach event on 5 calling it B => we see [4: A, 5: B, 6: C]
-> change start of recurrence to 5 => we see [5:B, 6:A, 7: A]
-> change start of recurrence to 7 => we see [5:B, 7:A, 8: A, 9:A]
-> delete detached event B on 5 => we see [7:A, 8: A, 9:A]
-> change start of recurrence to 3 => we see [3:A, 4: A]

closes #10873
fixes #10278
opw-665700

addons/calendar/calendar.py

@@ -1564,12 +1564,16 @@ class calendar_event(osv.Model):
             else:
                 real_event_id = calendar_id2real_id(event_id)
 
-                # if we are setting the recurrency flag to False or if we are only changing fields that
-                # should be only updated on the real ID and not on the virtual (like message_follower_ids):
-                # then set real ids to be updated.
-                blacklisted = any(key in values for key in ('start', 'stop', 'active'))
-                if not values.get('recurrency', True) or not blacklisted:
+                # update the real id unless it is a deletion of a virtual still to be recurrent
+                if 'active' not in values or not values.get('recurrency', True):
                     real_ids = [real_event_id]
+                    # if start change, update recurrent_id_date time of detached events
+                    if 'start' in values:
+                        _, _, new_time = values['start'].partition(' ')
+                        detached_ids = self.search(cr, uid, [('recurrent_id', '=', real_event_id)], context=context)
+                        for event in self.browse(cr, uid, detached_ids, context=context):
+                            detached_date, _, _ = event.recurrent_id_date.partition(' ')
+                            event.recurrent_id_date = detached_date + ' ' + new_time
                 else:
                     data = self.read(cr, uid, event_id, ['start', 'stop', 'rrule', 'duration'])
                     if data.get('rrule'):