mtp: support moving through get-put and increase concurrency to 2
this is a bit tricky. to support moving from one device to another properly we need KIO to take control since doing it inside the worker is exceptionally tricky with the current feature set. at the same time KIO is a bit defective and presumes that it can have a concurrent getjob and putjob running at the same time. to allow for that we now have 2 workers instead of 1. technically they could be deadlocking each other, practically they shouldn't because all the work is outsourced to the kiod though anyway and thus getting multiplexed