QmlProfiler: Make overview declarative

Using the TimelineRenderer's render passes we can as well draw the
overview in a nicer way without procedural painting.

Change-Id: I9464f10c52988a6af10c849878e678e4958a1057
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>

src/plugins/qmlprofiler/qml/MainView.qml

@@ -102,7 +102,6 @@ Rectangle {
         selectionRangeMode = false;
         zoomSlider.externalUpdate = true;
         zoomSlider.value = zoomSlider.minimumValue;
-        overview.clear();
     }
 
     function enableButtonsBar(enable) {