Support tent and tablet mode orientation sensors

Some laptops, especially laptop-convertibles with 360° hinges like Lenovo Yoga series, usually contain a pair of two sensors: an accelerometer mounted at the screen, and a gyroscope or a second accelerometer mounted at the keyboard.

iio-sensor-proxy could be extended to also provide the second orientation value, so use-cases like tent or tablet mode can be detected and supported.

It also has to be made sure that iio-sensor-proxy uses correct accelerometer for screen orientation purpose (not sure if it does already - my laptop has a gyro instead of 2nd accel). A case where there's only gyro(s) and no accels could also be considered - not sure if there's some hardware like that though.

[edit] I have played more and seems like the keyboard sensor isn't visible yet from Linux on my Yoga 2 Pro at all and the available gyroscope seems to come from the screen as well; however, some laptops do expose two accels already, see: https://github.com/alesguzik/linux_detect_tablet_mode so it could be supported by iio-sensor-proxy anyway.