Support for profiling multiple QML engines is incomplete

As QML engines can start and stop profiling on their own, without being directed to do so by the profiler, we need some functionality to stash and later merge profiling data on the client.

When profiling two QML engines it may happen that one of them exits and we don't necessarily want the other one to stop profiling at that point. What we should do is receive the profiling data from the exiting engine and keep the other one running. When receiving the second set of profiling data from the other engine we should merge the data sets.