PDF should be moved to group directory

[koppor]

Issues

• Multiple groups?
• Automatic groups?
• Do we need a flag "PDF file group"?
  
  Is this confusing? Because PDF does not contain domain semantics
  • PDF group is the "main taxonomy", the other groups are just additional "views" (such as reviewed, own papers, good conferences, keyword-search, ...)

[Siedlerchr]

You can already use the File Directory pattern to used groups name:
https://discourse.jabref.org/t/the-function-of-copy-linked-files-to-folder/1957/4

If you enter [groups:(unknown)] as a file directory pattern JabRef will put the linked files into a folder named after the group(s). If an entry is part of multiple groups, the group folder name is a comma separated list of group names.