[edit controller] Cancel RQL queries to be performed on entities to be deleted
Composite relation removal via the EditController trigger an entity
removal. Changes on such a to-be-removed entity must be cancelled as
they may trigger integrity errors before the deletion occurs.
Closes #8529868.
[edit controller] fix handling of removal of subentities from an edit form
In CubicWeb 3.18 the semantics of dropping a composite relation were changed.
It had before the effect of deleting the subentities.
Now the edit controller must handle this by itself (it was not adapted then by
mistake).
So if it detects the removal of a composite relation (on the browser side, the
user clicked on the "remove" action), the subentity is now explicitly deleted.
It was chosen not to change the form generation but the EditController:
- to ensure pre-3.18 forms backward compat
- to avoid the introduction of a specific entity-deletion API (RQL is probably
our best API for complex DB operations)
Related to #8529868.