Add inverse signal pin feature

[sengels]

Feature request:
I want to use the H-bridge BTS7960. It requires the DCC-signal and its inverse as input to create a proper DCC-output. I am currently using a special device to create the inverse signal, but it would be even more awesome if DCC++ could do this directly.

What would be required:

• a way in the api to add another signal pin
• obviously the signal generation at exactly the same time.

Until now, I failed to understand the code base properly to create a patch for myself, but I'll see whether I can come up with a patch in the next few weeks.

Thanks & merci!