Added &mut option for def file

[David-OConnor]

#138

I have found this necessary to set the default file name dynamically, in practice.

[fluxxcode]

Hi, thanks for the PR.

The default file name should be able to be dynamically adjusted via config_mut. Here is an example:

let mut file_dialog = FileDialog::new().default_file_name("some_file.txt");

file_dialog.config_mut().default_file_name = "some_other_file.txt";

Let me know if it works for you!

[David-OConnor]

I can confirm that works. Closing; ty!