Add handling of annotations

Annotations belong to a PDF. PDFs belong to a BibEntry.

1. A click on a BibEntry
2. I click on "add files"
3. JabMap creates sub nodes with with all PDFs files belonging to this entry
4. I click on a PDF
5. I click on "add annotations"
6. JabMap creates sub notes with all PDF annotations

Variants:

1. "Refresh annotations" - adds new annotations (and removes non-existing ones). NOTE: JabMap remembers which annotations have been deleted by the user before and does NOT add them again
2. "Refresh PDFs" - similar to "annotations"
3. "Inbox", where automatically new annotations, pdf files, and BibEntries flow in

In case a PDF belongs to more than one BibEntry, handling might geht strange, but we focus on this relation kind here. (This somehow refs https://github.com/JabRef/jabref/issues/8128, which describes "proceedings", which is a complete PDF containing multiple papers)

No files or tests are identified in the issue. Start by tracing the entry points for “add files” and “add annotations” and how BibEntry, PDF, and annotation nodes are represented; done means annotations can be exposed under a PDF, with the described refresh and deletion behavior accounted for.