If there are too many events it takes too long to draw the overview.
By drawing only part of the events in each paint callback and
scheduling multiple paints we can mitigate that. The result is that a
rough outline is visible immediately and the details are filled in bit
by bit.

Task-number: QTCREATORBUG-12341
Change-Id: Id94d914e2926be01cb1635fbd8814c4d2f57d23b
Reviewed-by: Kai Koehne <kai.koehne@digia.com>