Add bg-cancel action to ignore running background transforms

Close #4430

Example:

  # Implement popup that disappears after 1 second
  #   * Use footer as the popup
  #   * Use `bell` to ring the terminal bell
  #   * Use `bg-transform-footer` to clear the footer after 1 second
  #   * Use `bg-cancel` to ignore currently running background transform actions
  fzf --multi --list-border \
      --bind 'enter:execute-silent(echo -n {+} | pbcopy)+bell' \
      --bind 'enter:+transform-footer(echo Copied {} to clipboard)' \
      --bind 'enter:+bg-cancel+bg-transform-footer(sleep 1)'

src/options.go

@@ -1703,6 +1703,8 @@ func parseActionList(masked string, original string, prevActions []*action, putA
 			appendAction(actExclude)
 		case "exclude-multi":
 			appendAction(actExcludeMulti)
+		case "bg-cancel":
+			appendAction(actBgCancel)
 		default:
 			t := isExecuteAction(specLower)
 			if t == actIgnore {