Better handling of nospectrum/nodigital

[azonenberg]

• Replace nodigital/nospectrum args in ScopeApp/OscilloscopeWindow with bitmask so we can request any set of channels
• Add checkboxes to startup/connection dialog for initial channel set, allowing any of analog/digital/spectrum. Enforce at least one being checked.

[azonenberg]

Change of plans, just hide spectrum/digital channels at startup unless they're on already.