PLCgen: Things to do for first release

The PLCgen code generator is near a point where core functionality is feature complete. Next steps are more about extending and improving functionality. As such, a new phase in the development seems to be reached.

It's likely that some things are still broken or don't work as expected or should work differently. Finding these problems is more effective if other people can try the generator as well.

So the question is what should be done to reach this point. The following seems useful:

• Verify that #397 (closed) has no core features that must be in the program before a first release.
• Update precondition checker for features considered in #397 (closed), but that are now out of scope for the first release. (!689 (merged))
• Make the application usable from the GUI. (!690 (merged))
• Write some documentation how to use it and other useful information for this version. Mark the application as experimental / work in progress.
• Deal with the PlcFormalFuncInvokeArgOption, PlcFormalFuncInvokeFuncOption, and PlcMaxIterOption options of the program. (!697 (merged)) They should be implemented, or be ... They are postponed to #679 (in !675 (closed)).
• Ensure that remaining points in #397 (closed) are added to the follow-up tracking issue #679.
• Ensure that points that have come up (at least the TODO list currently hidden in the code) are added to the follow-up tracking issue #679.
• Add a plcgen ToolDef function. Implemented in !674 (merged).
• For %v3.0, close #382 (closed) and !357 (closed). Also delete the 382-add-plcgen0 branch as closing won't delete it. And remove the 'sticky' build for it in Jenkins. (We've put #382 (closed) in %v3.0 and added these steps there.)

For next steps after the first release, see #679.

Closes #397 (closed)