annotation and notes extraction and consolidation: improve

[rdiaz02]

From the README, but so I have it here for quick reference:
(https://github.com/rdiaz02/Adios_Mendeley#extracting-all-pdf-annotations-and-placing-them-in-an-org-mode-file)

• Trigger the extraction only for the PDF that is modified (and neatly insert the annotation in the proper place).
• Populate the annotation file only for files with annotations (not with useless links to PDFs that contain no annotations).
• Create the annotation file in the best way for helm-bibtex to understand.
• Use the approach in Note Taking with PDF Tools to generate that file for helm-bibtex.
• Figure out whether I want a single file with all annotations or a file per entry (preliminary tests suggest that a single huge file will lead to deals in helm-bibtex).
• Add the notes from Zotero itself to the org file(s) with annotations, or merge them into just one annote field. This way, I'd have a single place to do all and every search (helm-bibtex can search on additional search fields, but if you have several per entry only the last one is used).