It looks like the case where workers need talk to each other by
only knowing the type of the 'partner' does not exist in practice
anymore. With the now-common setup of a 'primary' worker that one
can introduce the 'lesser' workers to each other directly.

That's also conceptually more robust that picking a partner by
type only only from some 'pool' (all the workers in a runcontrol),
scales better (it e.g. is imaginable that a RunControl needs
more than one PortGatherer in complex setups where more than one
device is involved) saves a few cycles, and even removes the need
for workers to be qobject_cast-able.

Change-Id: Ib3d8c942c893d6c198d9813cce7df28ba3260ce8
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: hjk <hjk@qt.io>