Initial sensors backend for WinRT/Windows Phone

This provides the sensor plugin with support for accelerometer,
gyroscope, rotation sensor, orientation sensor, compass, and
ambient light sensor.

Change-Id: Ic91a6cef98e4011858552c5cd6407b494579fe17
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>