port to kio::workerbase

this fixes a number of crashes pertaining to poor state management. specifically error() and finished() may not be emitted multiple times and KIO will hard-assert that. the new workerbase completely removes this category of issue by moving error/finished from signals to return values forcing a conclusive singular state after a function has run.

ups the required frameworks version but keeps the compiler settings at the legacy version to keep the diff focused on the workerbase port.