QmlProfiler: Avoid race conditions when loading or saving data

We cannot assume that the model manager is still available when the
operation finishes. Also, accessing the QML model or the trace time
from the thread is dangerous.

Change-Id: I673c57c09490a0e3e2647f3197929eff1ce4ceb3
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>