Add 'focus' event

Can we find a better name? I have considered the followings.

* 'point', because "the pointer" points to the current item.
* 'shift', 'switch', 'move', etc. These are not technically correct
  because the current item can change without cursor movement (--tac,
  reload, search update)
* 'change' is already taken. 'change-current' feels a bit wordy and
  sounds wrong, 'current-changed' is wordy and doesn't go well with the
  other event names
* 'target', not straightforward

Close #3053

CHANGELOG.md

@@ -3,6 +3,20 @@ CHANGELOG
 
 0.37.0
 ------
+- New event
+    - `focus` - Triggered when the focus changes due to a vertical cursor
+      movement or a search result update
+      ```sh
+      fzf --bind 'focus:transform-preview-label:echo [ {} ]' --border --preview 'cat {}'
+
+      # Any action bound to the event runs synchronously and thus can make the interface sluggish
+      # e.g. lolcat isn't one of the fastest programs, and every cursor movement in
+      #      fzf will be noticeably affected by its execution time
+      fzf --bind 'focus:transform-preview-label:echo [ {} ] | lolcat -f' --border --preview 'cat {}'
+
+      # Beware not to introduce an infinite loop
+      seq 10 | fzf --bind 'focus:up' --cycle
+      ```
 - New actions
     - `change-border-label`
     - `change-preview-label`