Support node revisions

Yup, totally agree on that. I could have sworn that there's a "support revisions" issue in the filefield issue queue, but it's not here. So this is the official issue now, and as such, let's rename it to something that is recognizable at first sight.

Patches are highly appreciated, as would be a code analysis why this thing doesn't work while other CCK fields don't have any problems with revisions.

I ported dopry's imagefield patch to filefield - applied in commit 117907, so revisions work now.

Also, I found that when reverting from a revision with a non-empty filefield to a revision where the filefield was empty, the resulting copy of the original revision doesn't has the previous file filled in even if that revision didn't contain it.

But well, that's another bug, and deserves another issue. For now, it's time to enjoy revision support. Yay!