Better support for managing annotated PDFs

[koppor]

When viewing a PDF, I'd like to make highlightnings and comments. When sending around the PDF ("send as email"), I'd like to send the plain PDF.

I could solve that with manually copying the PDF and attaching the second one too (ALT+f), but when sending around, I have to manually remove the second PDF entry in the files key.

Solution possibility: Create a copy of the pdf in a temp dirctory, remove the comments there and sending that PDF as email (in JabRef).

[koppor]

Somehow refs JabRef#160

[koppor]

The driver of the following solution is a shared bibliography in a group of researchers.

Solution proposal:

• Two kinds of PDF files: "plain" (without comments) and "commented" (with comments)
• The file ending - comments.pdf or - comments USERNAME.pdf indicates that a PDF contains comments.
• JabRef offers the functionality to create a commented PDF (just creates a copy of the plain PDF in the user-specific file directory)
• JabRef displays a "comment" icon NEXT to the PDF icon if a commented PDF is available. (The commented PDF file itself DOES NOT get an extra row in the files field in the UI. The commented PDF, however, is stored in the files field in the BibEntry)
• JabRef does not display not available commented PDF of other users in the file field.
• In case more commented PDFs are available, JabRef lists the PDFs of other users as "normal" file
• A click on the comment icon opens the commented PDF, a click on the "normal" PDF icon opens the normal PDF.
• If only a commented PDF is available, JabRef also displayes the "comment" icon. The PDF icon is having a lighter color to indicate that the plain PDF is not available.

This issue is related to #571, because at the #571, the moving functionality itself and some icon handling is described.