Revert "ui/app,apps: unexport ChangeStage and Stage"

I found a convincing use case: stopping asynchronous activities
while paused. A follow up change will rename the stages and add
an example.

This reverts commit f9840b0963.

apps/gophers/main.go

@@ -185,6 +185,7 @@ func (a *App) run() error {
 						}
 					}
 				}
+			case app.ChangeStage:
 			case *app.Command:
 				switch e.Type {
 				case app.CommandBack: