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/options.go

@@ -609,6 +609,8 @@ func parseKeyChordsImpl(str string, message string, exit func(string)) map[tui.E
 			add(tui.Start)
 		case "load":
 			add(tui.Load)
+		case "focus":
+			add(tui.Focus)
 		case "alt-enter", "alt-return":
 			chords[tui.CtrlAltKey('m')] = key
 		case "alt-space":