LegacyUI: use IJ1's dialogs for opening and saving

This is more consistent with the legacy IJ1 experience. It will also
respect the value of the "Use JFileChooser to open/save" setting,
whereas the previous code always used JFileChooser which is
nigh-unusable on OS X.

See also:
http://fiji.sc/bugzilla/show_bug.cgi?id=780
http://fiji.sc/bugzilla/show_bug.cgi?id=781