Add :selected-file to file-chooser

[mschuene]

There is a swing "hack" of suggesting an initial filename for a file save dialog by calling
(.setSelectedFile FileDialog (file "blub"))
See also
http://stackoverflow.com/questions/356671/jfilechooser-showsavedialog-how-to-set-suggested-file-name

I think a :selected-file option for file-chooser would be convenient for this.
Also worth thinking of a better name like :suggest-name maybe

[daveray]

Sounds totally reasonable to me. I'm not sure when I can get to it, but patches welcome :)