Ensure consistency of "file linking" actions

[koppor]

• Remove icon on the left ("Automatically set file links for this entry Alt + F")
• Remove "Download from URL" @koppor got used to this button and wants to keep it.
• Add functionality to "Get fulltext": Auto: if not found, say "not found. please input URL . [OK] [CANCEL]";
  • For a single entry: Look up full text document in local folder, Look up full text document in web, Download document from URL (also check what Tools -> lookup full text document does)
  • For a bunch of entries: Synchronize (combination of find unlinked and automatically set file links)
  • Eventually rename "Get fulltext" to "Automagically add fulltext" to (i) state that both a local lookup and a web lookup is made and (ii) to distinguish it from "Quality -> Lookup fulltext documents" (which only does the remote lookup)
    See also Improve consistency of file related actions in menus and entry editor JabRef/jabref#190, which provides an overview on currently available file linking options.