File dialog functions
These functions present dialog boxes for requesting new and old files according
to platform conventions.
- request_old_file([prompt = string]
[, default_dir = DirRef])
- Prompts the user for an existing file. Returns a FileRef object, or None if the user cancelled.
- request_new_file([prompt = string]
[, default_dir = DirRef]
[, default_name = string])
- Prompts the user for a new file. Returns a FileRef object, or None if the user cancelled.