Add support for OpenRTX radios

[hmatuschek]

This requires a lot of work (in openrtx branch):

• [ ] Implement OpenRTX codeplug encoding/decoding
• [ ] Implement interface to OpenRTX radios
• [x] Implement M17 Channels
• [x] Implement M17 Contacts
• [x] Implement M17 RadioIDs
• [x] Fix YAML generator/serializer
• [ ] Fix GUI
• [x] Fix AnyTone codeplug generation to tolerate M17 elements.
• [x] Fix TyT codeplug generation to tolerate M17 elements.
• [x] Fix Radioddity codeplug generation to tolerate M17 elements.
• [ ] Update documentation

[jancona]

I just got a CS7000-M17, so let me know if there's anything I can do to help with this.