Playback slots are never pruned

[mrkline]

RadioPlayback::_slots and RadioPlayback::_frequencies are never removed - UntuneFrequency() just sets IsTuned = false

[richionizor]

I would argue that this is hardly a bug: the Playback slots are linked to the individual channel cards and are cleaned up on Dispose() - how many entries would we keep during a single session? 5? 10?
Since we also use the same path when a user disconnects from a single frequency I would hardly argue that this is an issue tbh.