fix wrong content of elem->path
second part of filecompletion extension
- replace LIST with CIRCLEQ to enable backward traversing
- 'SPACE' typed after a directory completion switches cwd back to pwd,
  previous to this no directories could be selected as arguments
- extend 'BACKSPACE' to be able to remove arguments
third part of filecompletion extension
- extend 'LEFT' to modify previous arguments
- 'LEFT' typed in an empty argument switches to the previous argument
  and deletes the empty one
- if the previous argument is a directory further switching is applied
  until a nondirectory argument is found (meaning you can't modify paths
  in between but only by deleting from the end)