[linux] Upgrade SDL1.2 to SDL2 for hotpluggable joysticks

This is a set of patches that cleans up some of the CJoystick mess and makes it use SDL2 which brings hotplug support without udev hacks.

This PR continues @topfs2' work (in https://github.com/topfs2/xbmc/tree/sdl2) of moving to SDL2 for linux joystick handling. It is also a followup of https://github.com/xbmc/xbmc/pull/5357 which cleans up some of the joystick mess which made the transition to SDL2 less painful.
ButtonTranslator.h is included by many other files. All those files will get the entire boost clutter which is not desired. This can slow down build times tremendously.