More convenient file dialog

I'm wishing for an easier-to-use file dialog, so started writing this wrapper. Got stuck on how you manipulate lists. The thing I am most confused about is how you handle the situation where one object can yield `GSList`s [of multiple types](https://developer.gnome.org/gtk3/stable/GtkFileChooser.html). Currently, this works for single-selection but segfaults for multiple selection.

Before I bang my head too much on this, thought I'd see if this is even a direction you want to support.

Seems reasonable, now that one needs to call `showall` anyway.