Provide a BC layer around 1.x helper methods

Problem/Motivation

While most of the API changed in 2.x in what regards implementing plugins and data storage, it's possible that other modules were interacting with the API just through the ::listUsage() and ::listReferencedEntities() helper methods (1.x).

These don't exist on 2.x, which would make those integrations break hard.

Proposed resolution

We could make the transition easier by providing legacy wrapper methods with the same name, but using the new API. This would allow those existing integrations to install 2.x without needing to update their code.

They might still want to update the code however, because the new count returned is not going to be the same (once now we are tracking all revisions and translations), but at least things won't break hard.

Remaining tasks

User interface changes

API changes

Data model changes