Safely close database connections and delete backends

Also fix NewClosure leak caused by disconnected object signals

src/tidal/tidalbaserequest.cpp

@@ -52,7 +52,7 @@ TidalBaseRequest::~TidalBaseRequest() {
 
   while (!replies_.isEmpty()) {
     QNetworkReply *reply = replies_.takeFirst();
-    disconnect(reply, 0, nullptr, 0);
+    disconnect(reply, 0, this, 0);
     if (reply->isRunning()) reply->abort();
     reply->deleteLater();
   }