Discovered channel interactivity

[stonier]

Victory Conditions

• Last used selections are saved on app exit and loaded on app start
• When a drop-down item is manually changed, any existing subscriptions are cancelled and a new subscription is fired up
• When the drop-down is populated the first time, a new subscription is fired up for the last used (fallback: first) item in the list
• When an active selection disappears, it is kept alive and remains selected and visible in the drop-down. Other discovered channels are listed after the active one.

For now, cancelling and creating subscriptions are merely a stub (print to the console).