add native file dialogs for mac OS

[flavius-t]

Supports #16

Hi, thanks for creating this project, I found it very helpful for use on Windows. I thought it would be a good idea to also extend the functionality to support native file and folder browsing dialogs for Mac OS. I added support for Mac via AWT's FileDialog class.

Changes

• added showMacFileChooser and showMacFolderBrowser methods to JnaFileChooser, which are now called by showDialog if the platform is Mac.

Demo Images

I put together a demo app to showcase the functionality on Mac, here are some screenshots:

file selection, single select

file selection, multi select

file save

folder select

[alanchenup]

@steos very useful ,please add to master @flavius-t thx