DirectorySizeJob: optimization: use localPath when available

Useful for trash:/, recentlyused:/, desktop:/...

This allows to use file:/ worker instead of resolving meta-url back to file for each file and directory. This is particularly useful for directories with many small files.

I.e a dir in trash:/ with 5000 small files in trash, getting its size will be night and day with or without this patch.

I took the opportunity to write more guidance for the use of KFileItem::localPath().

cc @dfaure @ahmadsamir