QmlProfiler: Make TimelineModel usable from QML

Make the relevant member functions Q_INVOKABLE and properly notify
about the height change when data is loaded or cleared.

Change-Id: I7c1ee70942617af2045b84a2d8f0ab2adef72f52
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>

src/plugins/qmlprofiler/qmlprofilertimelinemodel.cpp

@@ -101,6 +101,8 @@ void QmlProfilerTimelineModel::dataChanged()
     default:
         break;
     }
+
+    emit labelsChanged();
 }
 
 int QmlProfilerTimelineModel::bindingLoopDest(int index) const