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 blocs all user input events.
Instead make the code async.
Reported by Alexander Volkov: frameworks/kio@fdb8e14e (comment 380644)