File dialog functions

These functions present dialog boxes for requesting new and existing files according to platform conventions.

request_old_file([prompt = string] [, default_dir = DirRef][, file_types = list of FileType])
Prompts the user for an existing file. If file_types are supplied, only allows the user to select files matching one of the specified types. Returns a FileRef object, or None if the user cancelled.
request_new_file([prompt = string] [, default_dir = DirRef] [, default_name = string][, file_type = FileType])
Prompts the user for a new file. If file_type is specified, it should represent the type if file which is to be created; some platforms use this to restrict the list of existing files displayed. Returns a FileRef object, or None if the user cancelled.