Handle subclassing properly in stub files

[lars-reimann]

Is your feature request related to a problem? Please describe

#292 does not require the handling of subclassing. This is extracted into this issue. Tasks:

• List the superclasses of a class after the sub keyword

[lars-reimann]

Good news: The modifiers open and override are removed from the stub language. Therefore, this task is reduced to listing superclasses properly.