* Use [1:*] as indicator that range selection is in progress

* Fix review comments

* Add prompt for plugin selection

Update the selection buffer and file in-place:
- Append file path when one is added.
- Remember the last position in selection buffer at which the next filename
  is to be appended when a directory/context change happens (lastappendpos).
  So in case of a de-selection we can go back to that position and scan all
  the entries to add only the ones which are selected.