io/router,app: scroll a bit when reaching the end in a focus direction

List was recently changed to include an extra child at each end, to
automatically scroll when reaching the end of a focus direction. However,
if List includes unfocusable children that strategy may fail. This change
adds another fallback where app.Window will scroll a constant amount in
the focus direction, to reveal more children.

For https://github.com/tailscale/tailscale/issues/4278.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/router/key_test.go

@@ -285,7 +285,7 @@ func TestFocusScroll(t *testing.T) {
 	r.Frame(ops)
 
 	r.MoveFocus(FocusLeft)
-	r.ScrollFocus(image.Rect(0, 0, 15, 40))
+	r.RevealFocus(image.Rect(0, 0, 15, 40))
 	evts := r.Events(h)
 	assertScrollEvent(t, evts[len(evts)-1], f32.Pt(5, -10))
 }