This is useful when the files/directories lists are not cleared; then it is possible to define priorities between the files.
For instance, the skin "Default" will be shown only once, even if the directory "Default" exists both in the system and the user directories.
As the skin and translation loading functions does check both directories, there is no problem in doing that.
If set to "false", the previous list of files/directories won't be cleared when browsing another directory.
It will allow us to have file choosers that list files contained on different directories.
