Instead of a blunt delete() RunControl::initiateFinish() is triggered
by the closing of the application output instead. The rampdown process
is basically the same as stop() now, except for the other success
signal (new finished()) and the final self-destruction of the
runcontrol.

stop() itself triggers initiateStop() on all running workers in
parallel (before it was in the order of start). This gives
downstream complex worker combinations the flexibility to use any
order it wants by ignoring stop() on 'uninteresting' workers,
and centralizing rampdown e.g. in the main worker. That setup should
be rare in practice, but seems needed in some profiler cases.

Change-Id: I986a152a663754206709ed4df0d4568847afad17
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>