ui/app,apps: unexport ChangeStage and Stage

I'm not convinced the API is right. For exmaple, an event that
notifies a program when to save its state is both smaller and
might be sufficient.

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

ui/app/os_js.go

@@ -56,7 +56,7 @@ func createWindow(opts *WindowOptions) error {
 	w.w = newWindow(w)
 	go func() {
 		windows <- w.w
-		w.w.event(ChangeStage{StageVisible})
+		w.w.event(changeStage{stageVisible})
 		w.draw(true)
 		select {}
 		w.cleanup()