eca_content: Provide an addition and removal events for entity references

Problem/Motivation

It may be useful to have a specific event when

• An entity was added as reference to an entity reference field
• An entity got removed from a entity reference field

Not having such an event, one needs to manually lookup into the original values of an entity reference field, and filter out the ones that got added or removed by comparing against the current list of referenced entities. Such an event would make handling of such cases much easier.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes