Gets a text string from the user via a modal dialog, using parent as the parent window if it is specified. The dialog's title is title. The dialog's text field is labelled with message and initialized to init-val (but init-val does not determine the size of the dialog).
The result is #f if the user cancels the dialog, the user-provided string otherwise.
The style argument is provided for future extensions. Currently, style must be the empty list.