Add order_by parameter in directory listing (set to -modified_at when paste from clipboard)

Files will be sorted according to order_by parameter if exists. If no this parameter is not
given, files will be sorted as before, by its str representation (name or original_filename).
Files can be sorted by more than one field, that have to be set separated by commas.

After pasting a file from the clipboard, the directory listing is called with this parameter
set to -modified_at. Therefore, the pasted element could be more easily selected.