Execute "thunar_file_is_empty_directory" in separate thread
thunar_file_is_empty_directory is used to check if an "expand arrow" needs to be shown when expandable folders are enabled in list-view.
The method is doing file IO, and as such can lead to performance problems on slow drives. In order to not freeze thunar in such cases, thunar_file_is_empty_directory should not be called directly, but in a separate thread, using ThunarJob
Related issue: