Hide separator next to preview window at 'next' position when decidable

Decision must not depend on threshold chain resolution
(computePreviewSize consults noSeparatorLine); separator is hidden only
when every spec in the chain displays a 'next' preview window with a
border line facing the input section. Otherwise err on the side of
showing it.

- Without active previewer, preview position is resolved from first spec
  alone, so 'next' in a threshold alternative does not create input
  window and cannot hide the separator
- Empty preview command is undecidable; preview(...) action can display
  ad-hoc window at 'next' position at any time
- change-preview-window relayouts or repaints when separator visibility
  changes even if the active spec is unchanged, without dropping preview
  window forced by preview(...) action

CHANGELOG.md

@@ -11,6 +11,9 @@ CHANGELOG
   # Separator shown; no border separates the input section from the list section
   fzf --style full --input-border none --header foo --no-header-border
 
+  # No separator below the border of the preview window at 'next' position
+  fzf --preview : --preview-window next
+
   # Conversely, separator is now shown when the input border does not draw
   # a line facing the list section
   fzf --input-border bottom