Figure out a design for slices/VFOs

[arodland]

We could do one hamlib server per slice, which basically works already, but we probably also want to be able to map slices onto hamlib VFOA/VFOB so that people can make sensible use of split mode. Defining that mapping is a bit of a UI challenge.

Also the hamlib concept of VFOs is a bit complex. It has a concept of "A" and "B" VFO (actually up to A through T, though some clients might not be able to deal with that... point being, 4 slices can be accommodated that way if a user really wants), and also "Main" and "Sub" VFO. I think that Main and Sub function as changeable aliases, but it's never been 100% clear to me.