Replaces the MultiThreadEnabled checkbox and the ProcessorCount default
with a single MaxConcurrentDownloads value where 1 means serial - the
behaviour Libation had before parallel downloads existed.
The old pairing could not express 'off' at all: the bool was hardcoded
true in the view model constructor and the int setter clamped to a
minimum of 2, so there was no way back to one-at-a-time downloads.
Collapsing both into one value makes that state unreachable rather than
merely fixed, and removes the risk of the two settings disagreeing.
Bounds live in Configuration as named constants: minimum 1, default 3,
maximum 10. Audible throttles license requests, so the default is
deliberately conservative and the cap keeps users from choosing a number
that produces license denials instead of speed.
WinForms swaps the 'Parallel downloads' checkbox for an 'At once:'
spinner that reads its bounds from those constants.
Allamagoosa