New protocol decoder: Stepper motor

Another protocol decoder that hasn't had an official announce yet is stepper_motor, a PD which decodes speed and position of a stepper motor.

The PD takes a "step" and "direction" pin as input.

You can select either steps or millimeters as unit for the decoder output.

As usual we also have a few sample dumps in the sigrok-dumps repository, and a few test-cases in sigrok-test.

The decoder was contributed by Petteri Aimonen, thanks a lot!