Event transitions in plcgen

After finishing the expression conversion and generating code for setting the initial state in #418 (closed), performing event transitions are next.

Initial steps are done in #558 (closed) (step 9 of the plan, "collect automata in CIF"), next steps are steps 10 ("perform edges") and 11 ("synchronize automate on event"). Steps 12 (monitoring support), 13 (evaluate guards), and 14 (perform updates) are nearby too.

Step 15 (channels) is a next logical step, but let's only make sure this fits in the general pattern, and nothing more.

@jverbakel Feel free to chime in

Addresses #397 (closed)