Support Zs3/Zs3v as form signal

Currently, it is not possible to specify whether a Zs3/Zs3v symbol is a light signal or a form signal, so the implicit assumption is that everything is a light signal. A flag to specify this should be added and it should be implemented in the importers/exporters (e.g. PlanPro uses the `<Geschaltet>` element of the respective signal object for that)