Previously BKE_appdir_folder_default would return a path
trailing slash. This funtion is only used with SpaceFile's params->dir,
where dir is normally a path with trailing slash. This change makes the
default path consistent with other uses of params->dir.

This fixes previous/next navigation where stepping "back" into the
default folder was recorded as a newly visited folder, whereup the next
folder list was reset and the previous folder list became [default
folder]->[default folder with trailing slash].