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/collection/collectionmodel.cpp

@@ -130,7 +130,6 @@ CollectionModel::CollectionModel(CollectionBackend *backend, Application *app, Q
 }
 
 CollectionModel::~CollectionModel() {
-  backend_->Close();
   delete root_;
 }