Add Joystick support for Qt

Currently one of the only (easy) cross platform methods for getting access to joystick devices is by using the SDL library with its Joystick API and mapping these events to Qt Signals or custom events. To do this, you need to allow the SDL event loop to run which you have to manage with a QThread in order that the Qt Event loop isn't blocked.
What would be good is if the joystick events could be gotten through Qt into the Qt event system like keyboard and mouse and used like any other input device - without having to fit a third-party library into the Qt paradigm.