Start JabRef and "example library"

JabRef offers Citations:

One can select different providers:

("Open Citations" should be available soon - #15023)

The Web search has "Search pre-selected".

For the citations, we want to have "Search all" - and then union the result. - Use org.jabref.logic.database.DatabaseMerger#merge(org.jabref.model.database.BibDatabaseContext, org.jabref.model.database.BibDatabaseContext, java.lang.String) for merging two libraries (results from the fetcher)

Task:

1. Add "All" as first entry to the combo box
2. If selected: Use all citation fetchers and union the result. Use the existing backend.