PLCgen: Remove app framework instance requirement outside main app class.

Eliminate imports of app.framework classes outside the "main" class, that need a proper application instance to work.

A scan of the imports resulted in

• Warning output is used, there is a replacement in common.java.output. (!665 (merged))
• org.eclipse.escet.common.app.framework.Paths is used in the PLC writers.
• At various places app.framework exceptions are used, but that should be fine.
• plcgen.options classes use the option construction classes. As these are considered to be part of the "main" app, this should be fine as well.

Addresses #397 (closed)