saving as braille seems broken on macos

[sthibaul]

As discussed on https://www.freelists.org/post/liblouis-liblouisxml/Automatic-translation-of-MusicXML-to-braille-music,16 it seems saving as braille is broken on MacOS: the save-as dialog box only shows the xml choice, and not the others.

[judithhinlung]

Yes unfortunately the Swing Filechooser is not very compatible on OS X, so the file type options are not displaying correctly. I am working on a pull request to fix this issue as well as make Freedots friendlier to use on Mac.