* Similar to parameter's of type "string":
  Has a text entry and stores a string preference
* Additionally offers a button to show a file chooser dialog
* The node's content is the initial (default) file path.
* Relative paths will be considered relative to the extension's .inx
  file. This allows to reference files/folders shipped with the
  extension in a portable way.
  The stored value as well as the parameter value passed to the
  script interpreter will always be absolute, though.
* The attribute "mode" controls what type(s) of paths the file
  chooser allows to select. Valid values:
   - 'file', 'files', 'folder', 'folder' (pick existing items)
   - 'file_new', 'folder_new' (create a new file/folder)
* Note that manually entered values will be passed as-is without
  checking for existence.