#679 PLCgen: Introduce edge selection variables by automaton in event transition code

In transition code, the selected edge is remembered between edge selection and taking the edge.

Currently, each event creates new variables to remember the selected edge. Besides using many variables, mapping selected edges back to an automaton can be complicated.

This patch creates one edge variable for each automaton that need to have the edge remembered, and uses that variable for all events. This reduces the number of created variables, and keeps a 1-1 mapping between edge variable and the associated automaton, simplifying mapping selected edges back to an automaton.

Addresses #679