Joystick: Only send joy events when focused.

This changes makes it so that you only receive joystick (and implicitly
gamecontroller) input events when your application has keyboard focus.
If you'd like to still receive events when your application is in the
background, set the SDL_JOYSTICK_ALLOW_BACKGROUND_EVENTS hint to "1".

This fixes http://bugzilla.libsdl.org/show_bug.cgi?id=1892