QmlProfiler: Guess the trace end time if we don't receive EndTrace

Task-number: QTCREATORBUG-11952
Change-Id: Ie55b616a1fc54e0cccac5560ec66fb7cc496c8e2
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>