Using native file choosers whenever possible. cleanup.

 - Had to resort to a UX workaround as FileDialog on MacOSX does not
support simultaneous picking of a folder and a file, therefore
when on Mac, the dialog has 2 buttons, one to select files, one to select
directories.

 - On Windows and Unix it should work with a single button, need to test now.

 - Got rid of JFileChooser instances, now using only the FileChooserHandler
api, much simpler IMO.

 - Don't go to Search Tab if you cancel opening a torrent.

 - NPE fix.