[BatchRenameJob] Use KJob::Items when reporting the progress info
A batch rename job is usually fast (direct renaming), calculating the number of individual files and dirs would mean stat'ing which would make things slower for little gain. Instead, per dfaure's suggestion in the bug report, use "items" as a neutral term to represent files and dirs, this way the notification message would be "correct" when reporting "Job Finished, x of y items".
Also note that if the user is using Dolphin, or other KIO based file managers, that data is already visible in the status bar when the user selects the files/dirs to be renamed.