QmlProfiler: fix progress bar visibilty for loading traces

Move the main part of the loading work into a background thread
and show the standard progress indicator for the "reading file"
part of the load operation.
The load operation can be canceled now.

Change-Id: I4cb3b762072ab4a0665dcf9d4a39d6d6630d22e8
Task-number: QTCREATORBUG-11822
Reviewed-by: Ulf Hermann <ulf.hermann@theqtcompany.com>

src/plugins/qmlprofiler/qmlprofilermodelmanager.h

@@ -137,6 +137,7 @@ signals:
     void stateChanged();
     void progressChanged();
     void dataAvailable();
+    void loadFinished();
     void saveFinished();
 
     void requestDetailsForLocation(int eventType, const QmlDebug::QmlEventLocation &location);
@@ -162,6 +163,7 @@ public slots:
     void load();
 
     void newTimeEstimation(qint64 estimation);
+
 private:
     void setState(QmlProfilerDataState::State state);