macosx: implements user choice to autosave volume on exit and changes volume slider range to 0 ... 200%

this adds the configuration variable and also the respective gui modifications for
simple prefs dialog. Also, this commit changes some spacing in gui to be consistent
between the different tabs.
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>
(cherry picked from commit c2e39623ffa829913be56ad2030f507ba0470d35)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>