QmlProfiler: Rename QmlEvent::startTime to "timestamp"

As many events are instantaneous and we're going to drop the duration
property soon, this is more fitting.

Change-Id: I6e13dd076a5b9df16aed44bf9f631ea5760e9cbf
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/plugins/qmlprofiler/qmlprofilerrangemodel.cpp

@@ -77,7 +77,7 @@ void QmlProfilerRangeModel::loadData()
             continue;
 
         // store starttime-based instance
-        m_data.insert(insert(event.startTime(), event.duration(), event.typeIndex()),
+        m_data.insert(insert(event.timestamp(), event.duration(), event.typeIndex()),
                       QmlRangeEventStartInstance());
         updateProgress(count(), eventList.count() * 5);
     }