QMLProfiler: Use scene graph for painting events in timeline

By using the scene graph we can retain the geometry for events in
the timeline on the GPU and potentially speed up the rendering for
large amounts of items.

Change-Id: I2cfbb8ef4ebc7b56f1977ec1facb4f2e7f2002ee
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>