AudioDevice:
- for now every AudioDevice must use the config cache (needed for correct indexing)
- add another key to the config cache indicating whether the device was removed by the user (KConfigBase::deleteGroup doesn't do the job :( )

PhononServer:
- for ALSA virtual devices use the description text as unique ID
- for all devices the uniqueId needs to end with "playback" or "capture"
- handle multiple soundcards that we have no possibility to distinguish (except for the card number, but this number only depends on which card was found first)
- simplify reading audio device properties a bit by removing the type separation
- add functions to allow the KCM to remove devices
- use a 50ms timer to delay device relisting, this get's rid of some unnecessary full relistings when a single device removal/addition calls our slot multiple times