Fix EZP-23412: setting a section indexes content, even with DelayedIndexing

https://jira.ez.no/browse/EZP-23412

Updating an object's section automatically indexes the object, not respecting the `DelayedIndexing` flag in `SearchSettings`.

This "defers" the operation to `eZContentOperationCollection::registerSearchObject`, which handles all the necessary work.

Tests: manual.