Add handling of annotations

[koppor]

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

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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 JabRef/jabref#8128, which describes "proceedings", which is a complete PDF containing multiple papers)