Request a repaint from the TimelineRenderer if endTime changes

Previously the timeline was only repainted if the selected start time
changed. If the start time is != 0 changes of endTime unfortunately
imply changes of startTime most of the time. This is why the
effect was mostly invisible.

Task-number: QTCREATORBUG-10707
Change-Id: I21a86a4248c949f5ad630e6c5a30f8caedcbfe7e
Reviewed-by: Kai Koehne <kai.koehne@digia.com>