I don't understand the magic but sometimes pad buttons are detected as buttons
sometimes as axis. Create an option so people can test both and hopefully find a working solution
Note: you need to restart the plugin to take the option into account
Note2: fix dual pad init too
Related issue #938 and #414
+ Isolate every SDL code paths and replace any call to them by call to a generic class "GamePad" of which JoystickInfo is now a child.
+ Now backends can be added by inheriting GamePad generic class.
+ There is just one function change which is redundant with next commits but otherwise commit will not compile(which is more evil).