Add mode for viewing 'git annex list' output

The output is dumped into the buffer without much processing. One
possible improvement would be to color present and absent files
differently.
Modify file actions for magit-annex-list-mode

- Bind `magit-annex-file-action-popup' to single key for slightly
  quicker access.
- Teach `magit-annex-completing-file-read' to use file from
  'magit-annex-list-mode' line as default value.
Merge branch 'n/list-files' into next