Don't use a nested eventloop when renaming files

Using a nested eventloop (started by KJob::exec()) meant that in case of
file names conflict, the KIO::RenameDialog that is shown can't be accessed
at all, as the eventloop of the job blocks all user input events.

Instead make the code async.

Reported by Alexander Volkov:
frameworks/kio@fdb8e14e (comment 380644)