Use SensorProxy to detect accelerometer

[davidmhewitt]

Fixes #116

Instead of assuming that a touchscreen implies an accelerometer, use SensorProxy to ask if there is an accelerometer.

This is the bus that Mutter watches for rotation changes, so if we use this, we're guaranteed to show the switch if auto-rotation is supported.

I don't have a snazzy auto rotating laptop though, so can someone that does check that the switch still appears for them? :smile:

[cassidyjames]

I know @danrabbit has a convertible Lenovo Yoga

[danirabbit]

I can confirm that indeed the switch does show here!

@cassidyjames can you confirm that you don't get the switch on your XPS?

[cassidyjames]

@danrabbit correct; the switch does not show on my Star LabTop (no touch nor accel) nor on my Dell Precision (touch but no accel).