FileChooser (CommonDialogs) needs more features

- on Mac it can be a standalone window or a sheet attached to a window
- should invisible files be allowed to be shown/selected
- besides the title on Mac there is also a message (explaining what the purpose of the FileChooser dialog is: ex "Select image to edit") and prompt for the buttons ("save" and "open" are the defaults)
- allow or not to create directories (when saving files users may want to be allowed to create new folders)