Replace "Database" by "Library" in the translation

Following the discussion of https://github.com/JabRef/jabref/pull/1988, we decided to replace "Database" by "Library". This also has huge impact on the code (BibDatabase vs. BibLibrary)

Reasoning: 

1. Avoiding confusion with [SQL databases](https://help.jabref.org/en/SQLDatabase)
2. To be in line with [other bibliography management tools](https://github.com/JabRef/jabref/wiki/Alternatives).
