Currently there's no way to test or demonstrate SFML's sensor API so this adds an example for that - a simple ball rolling around based on the device rotation and movement, with a compass in the corner

As I was primarily testing on iOS I also fixed two major issues there:

• The gravity sensor was completely unimplemented
• The magnometric sensor was the raw value which isn't remotely accurate, there's a calibrated alternative which it's been replaced with

There's still a few other things noticed during my testing but don't want too creep the scope too much so have left them out for the time being:

• Android and iOS report opposite values (worked around in the example)
• Android sensors seem to lag quite a bit
• Values are reported relative to the device, so rotating the device gets confusing, especially without SFML exposing the device orientation outside of deducing it from the window size, for this example I have just locked it to portrait orientation