#583 Add url icon to new created field

[ph]

It's a nice feature that JABREF stores the url for obvious reasons. However, sometimes you want the url to show up in your TeX document reference list. If you use a bibliography style that does not "support" the url there is no problem, but certain styles do. In that case you can put the url in a new created field called for example myurl, which is not supported by any bibliography style. The big disadvantage of this is that the "url icon" in the third column is gone and you cannot go the article by a simple click. So, at least I think, it would be nice if you can put the icon at the new created field.