Improvements to device manager

- Mount and unmount devices in lister thread
- Safely close watcher and backends for devices
- Enable abort loading device
- Fix MTP connection

src/playlist/playlistbackend.cpp

@@ -84,7 +84,6 @@ void PlaylistBackend::Exit() {
 
   assert(QThread::currentThread() == thread());
 
-  Close();
   moveToThread(original_thread_);
   emit ExitFinished();