notify if previously configured devices were not found

[ronso0]

Mixxx should complain when previously configured devices were not found.

Unfortunately, when reading the sound config file, SoundManagerConfig::readFromDisk only keeps devices that are currently available. Those devices are used in SoundManager::setupDevices, and that throws a warning only if devices found earlier are not available anymore (or some other error occured), but NOT if the config can't be applied due to missing devices.

Reproduce:
* configure Mixxx with an external soundcard
* close Mixxx

* next time you start Mixxx, 'forget' to attach or switch on the device
* Mixxx will select no (or the first device?) and warn only if no output
  is set up at all

There is also an experimental branch that allows Mixxx to directly connect

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/10817