Make sure we never have a nullptr track in getTrack

And comment out all threadsafecalls debug output
Make sure qml can't delete our playlists
Stop and clear the cache manager before quitting
Fix cachemanager.

Releasing is only needed if we previously call add_ref (which happens
only in the QSpotify constructors) or if we create the spotify object
which does not happen for the objects we manage.
Allow to build for Sailfish Emulator