New protocol decoder: timing

libsigrokdecode now supports another protocol decoder: timing. This has been supported for a while already actually (and is part of libsigrokdecode 0.4.0), but hasn't seen an official announce yet, so here goes.

The PD works on a single logic channel and shows (for all transitions) the time between a transition and the next one.

As usual we have a few test-cases in our sigrok-test repo to ensure there are no regressions later on as the backend code and/or PD change.

The decoder was contributed by Baruch Even, thanks a lot!