Add some means to configure VFOs

[hmatuschek]

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