Tracing: Move event storage out of trace manager

This is a step toward making the trace manager non-virtual. It should
just juggle the storages for types and events, and manage the threads to
access them. It doesn't need to know what exactly it stores.

Change-Id: I45093c60d8ae921e68aeb09bd48d24d5877ce306
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qmlprofiler/qmlprofilerstatisticsmodel.cpp

@@ -112,8 +112,10 @@ void QmlProfilerStatisticsModel::restrictToFeatures(quint64 features)
         notesChanged(QmlProfilerStatisticsModel::s_invalidTypeId); // Reload notes
     }, [this](const QString &message) {
         endResetModel();
-        emit m_modelManager->error(tr("Could not re-read events from temporary trace file: %1")
-                                    .arg(message));
+        if (!message.isEmpty()) {
+            emit m_modelManager->error(tr("Could not re-read events from temporary trace file: %1")
+                                        .arg(message));
+        }
         clear();
     }, future);
 }