[BLUEJ-9] Open/Save dialog should be native on Mac OS X

The open/save dialog should be native on MacOSX. The native dialog has a lot of features and behavior that Bluej's doesn't \- I won't list them all because there is no need to reinvent the wheel. I don't know how to implement that, but I know it is possible. Eclipse and jAlbum are both java based and multi-platform and use the native dialogs.

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