#935 Make PLCgen transition generator more flexible

The current fixed split in trying uncontrollable events as often as possible, and then trying controllable events as often as possible is too crude.

• Looping over all (un)controllable events each iteration will eventually be replaced by more fine-grained looping, by adding event sequencing.
• Trying to perform one or more events must be movable to separate POUs, to avoid having a large body of code in the main program.

Rather than coding all these possibilities in the transition generator, it now takes a list of lists of CIF event transitions to eventually perform in one POU. The inner lists are the sequence of events to try in one iteration of a loop. The outer list allows to have several loops in one POU.
The generator converts each inner list CIF event transitions to a PLC statement sequence. The loop itself is not generated, and in fact not at all relevant to the transition generator, as before.