Discuss how to leverage E_USER_DEPRECATED

Problem/Motivation

We want to show users of APIs that certain aspects of APIs are deprecated. E_USER_DEPRECATED is the perfect tool for that,
as it actually allows people to see the errors.

Points to discuss

* How to leverage it properly
* How to integrate it into tests: #2488860: Bring phpunit bridge into drupal and use it for unit tests and simpletest to handle Deprecation is one related issue

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes