Thanks for the hint! I played a bit around with this example and indeed it is working, BUT:
I added the file-loaded signal and then it begin to behave different than inside gmoccapy code (user command file).
As a VCP I only get this message when I open another file.
But inside gmoccapy I get the message on every reload (reload button click).
And the message graphics-gcode-properties I don't get at all in a VCP.
