Suggest-name option in file-chooser

I added option for default file name to file chooser widget.
Option is string and it uses swing trick from http://stackoverflow.com/questions/356671/jfilechooser-showsavedialog-how-to-set-suggested-file-name

Hopefully I didn't miss anything as I am new to clojure and Seesaw.
I still have to figure out, why current directory is not changed, but I decided to create pull request, so we can discuss what should I improve and other possible mistakes.
Would `:selected-file` be a better name since this can also be used to set the initial selection to an existing file?