EZP-22466: advanced object relation list editing

> Fixes https://jira.ez.no/browse/EZP-22466

Two bugs here.

### Nested publish operations
When a content is edited through an "advanced" object relation list attribute (with `AdvancedObjectRelationList=enabled`), one publishing operation is started for each edited related object. The addition of a global transaction around publishing, plus 60f7a51a7de6b46921995a8b337d7a22339d58a5, messed things up. bcc1bfae7ca8afe1c4ee81647eb2ca81e464f489 fixes this by implementing a stack of nested operations.

### Regression in `eZObjectRelationListType`
24d134d0625b194b8b3e3c4d74786c9bfe1baa66 changed the type's validate method in a way that prevented data required for embedded editing from being fetched. Fixed by changing the way this data is fetched a bit.