apps,ui/app: delete ui/app.Window.Ack

Replace it with a dummy event send on the synchronous event channel;
a bit of cleverness for a simpler API.

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

apps/hello/hello.go

@@ -52,7 +52,6 @@ func init() {
 				w.Draw(root)
 				faces.Frame()
 			}
-			w.Ack()
 		}
 		if w.Err() != nil {
 			log.Fatal(err)