Make PLCgen the recommended CIF PLC code generator

This issue is to address a point from #679:

• Make the old PLC code generator deprecated, making the new PLC code generator the recommended PLC code generator. Do mark the untested targets as experimental (all but S7 and TwinCAT).

For %v6.0, TwinCAT, S7-300 and S7-400 will be declared stable, and the rest experimental. The old PLC code generator will be marked as deprecated.

Addresses #679