Add some means to configure VFOs

[hmatuschek]

trafficstars

Although never requested and thus likely rarely used, this feature is present in all radios. Adding VFOs requires:

• [ ] A representation within the codeplug
• [ ] A YAML representation (i.e., parser & serializer)
• [ ] Adding it to the GUI.
• [ ] Updating the manual
• [ ] Implementing it in existing devices:
  • [ ] AnyTone D868UVE
  • [ ] AnyTone D878UV
  • [ ] AnyTone D878UV II
  • [ ] AnyTone D578UV
  • [ ] BTECH DMR-6X2UV
  • [ ] TyT MD-380/390
  • [ ] TyT MD-UV380/390
  • [ ] TyT MD-2017
  • [ ] BTECH DM-1701
  • [ ] Radioddity GD-77
  • [ ] Radioddity RD-5R
  • [ ] OpenGD77