Copy "complete" BibEntry to a new database

[oscargus]

In both the BIB from AUX generator and in JabRef#1630 it would simplify things if if was possible to copy a complete entry from one database to another, complete in the sense that relevant crossref and strings are included. crossrefs are handled, at least one level, but strings are not. Given that such a mechanism was provided, what would the best way to implement it be?

newDatabase.insertCompleteEntry(BibEntry, oldDatabase) (where the BibEntry is not cloned, but the actual BibEntry in the oldDatabase)
oldDatabase.insertEntryIntoDatabase(BibEntry, newDatabase) (where the BibEntry is not cloned, but the actual BibEntry in the oldDatabase)

[oscargus]

Some progress in JabRef#1677 although it more focus on creating a complete sub-database.