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

src/tui/tui.go

@@ -92,6 +92,7 @@ const (
 	BackwardEOF
 	Start
 	Load
+	Focus
 
 	AltBS