QmlProfiler: When clearing, set the trace duration to 0

That is what the code in the main view expects of an empty trace and
it's also logically more coherent.

Task-number: QTCREATORBUG-11833
Change-Id: I78b90ca5332bfb004cabbb32c4eb3e7e83fd7187
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>