Remove of SlaveBase and subclasses
Open, Needs TriagePublic

Description

KIO::SlaveBase needs to be folded into KIO::WorkerBase, which it currently only bridges.
Can stay internally for a while and just be unpublished as public API though.

Other subclasses TCPSlaveBase & ForwardingSlaveBase can be dropped directly.

See https://invent.kde.org/teams/frameworks-devs/kf6-workboard/-/issues/45#note_712348 for usages left:

kup
nfs (kio-extras)
fish (kio-extras)
info (kio-extras)
bookmarks (kio-extras)
about (kio-extras)
bluedevil
fonts (plasma-workspace)

Created respective subtasks for it (and did some MRs for info & about)

SlaveBase is now internal, can be cleaned up at any point