Make workerbase.h independent of slavebase.h

To ensure already there are no hidden dependencies from including the
header of the deprecated class.
Using forward declarations where possible, so some consumers might need
adaption and do more explicit includes.

Needs some intermediate hack for duplicating the declaration of
KIO::unsupportedActionErrorString() in preparation of final move to
workerbase.cpp, given consumers of only slavebase.h still need to also
see it.