QmlProfiler: make sure indices in SortedTimelineModel are correct

In the rare case that a new item is not appended but inserted in the
middle of already existing data, the end times were confused. On that
occasion, also add a note explaining how the indices work,

Change-Id: I587b8285cd5482a9ffb1592302b442192e5944b8
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
QmlProfiler: Fix logic when selecting previous event

Events should be walked in the order of their start time, not end time.

Change-Id: Id9f9746958dbfbdc28424c34b3ca1f5e4b9ff373
Task-number: QTCREATORBUG-12634
Reviewed-by: Kai Koehne <kai.koehne@digia.com>