#1055 PLCgen: Construct a support class for creating a POU.

We're going to have more POUs, so some effort in abstracting from the details of creating it seemed useful.

It starts with eliminating some name confusion between variables in the POU (ie PLC context) and in the expression generator. I decided that "temp variables" are a PLC concept, and that PLC "local variables" is a non-PLC concept (since it's scope is the same as temp variables, the difference from the latter is persistence).

It's basically splitting the method of creating the transition POUs into a "general POU build" part and the specific code for transition POUs.
This is a small-effort patch, there are more things you can move to it (eg moving scratch and local variables from the expression generator). That might happen at some other time.

Addresses #1055 (closed)