Follow-up to #13020

We had an internal developer discussion:

• "Add entry"
• "Add entry using..."

And rename variables accordingly - should be easy with IntelliJ refactoring feature.

For the first one, we neglected "Add default entry" and "Add entry using defaults" (too long)

We think, we don't need an architectural decision record.

Sorry for the extra effort -- but naming is hard in computer science: https://martinfowler.com/bliki/TwoHardThings.html

(copied from https://github.com/JabRef/jabref/pull/13020/files#r2071452079)