change-preview-window to take multiple option sets separated by '|'

So you can "rotate" through the different options with a single binding.

  fzf --preview 'cat {}' \
      --bind 'ctrl-/:change-preview-window(70%|down,40%,border-horizontal|hidden|)'

Close #2376

CHANGELOG.md

@@ -7,6 +7,10 @@ CHANGELOG
     - cf. `preview(...)` is a one-off action that doesn't change the default
       preview command
 - Added `change-preview-window(...)` action
+    - You can rotate through the different options separated by `|`
+      ```sh
+      fzf --preview 'cat {}' --bind 'ctrl-/:change-preview-window(right,70%|down,40%,border-horizontal|hidden|right)'
+      ```
 
 0.28.0
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