Add back crossref support for notes to candidates

[bdarcus]

This was an oversight in #603 that needs to be fixed.

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Not sure if this should be incorporated somehow, or where, but this adds the "N" prefix to candidates whose crossref has a note.

(defun citar-xref-notes ()
  (let ((all-keys (mapcar (lambda (x) (nth 1 x)) citar--candidates-cache))
        (note-keys (citar-file--keys-with-file-notes)))
    (remq nil      
          (mapcar (lambda (key)
                    (let* ((entry (citar--get-entry key))
                           (xref (citar--get-value "crossref" entry)))
                      (when (member xref note-keys)
                        key)))
                  all-keys))))
  
(setq citar-keys-with-notes-functions '(citar-xref-notes citar-file--keys-with-file-notes))

(As an aside, is there a better way to get a list of all keys?)

Originally posted by @localauthor in #603 (comment)

[bdarcus]

I think, @localauthor, that #628 will fix this.