- reduce 4 spaces between help columns as it's too wide for handhelds
- support `]` as prompt in nav-as-you-type-mode when filter is empty
- update plugin docs

Since 75061367be we only need to fill one %c

* Cleanup unused MSG_

* Print only plugin keys

* Speed up preparing keys prompt

* Show prompt for bookmarks

* Don't error when user interactively pressed non-existent bookmark key

* Make help page more readable

* Always use 2 columns only

* 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.