#852 User icons

[Romko]

It would be nice if the user might be given a possibility to attach his own icons to "files" either by browsing the machine's hard disk or by contributing to the existing JabRef icon repository. Because the attached "files" might be of very different nature, not only the documents, but also various URLs to different indexing and reviewing services, such as Mathscinet, Zentralblatt, and many others, each enjoying its own icon.