[Don't merge] Try to receive life-cycle notifications

[hironytic]

The beginning of the story is this.

With this PR, logs below ware printed out to console. (I tried it with iPadOS 11.6 on simulator)

When app launches,

[sceneDelegate] sceneWillEnterForeground
<scene> sceneEnterForeground Notification
<app> willEnterForeground Notification
[sceneDelegate] sceneDidBecomeActive
<scene> didActivate Notification
<app> didBecomeActive Notification

Then show app-switcher,

[sceneDelegate] sceneWillResignActive
<scene> willDeactivate Notification
<app> willResignActive Notification

Switch to another app,

[sceneDelegate] sceneDidEnterBackground
<scene> didEnterBackground Notification
<app> didEnterBackground Notification

Switch to this app again,

[sceneDelegate] sceneWillEnterForeground
<scene> sceneEnterForeground Notification
<app> willEnterForeground Notification
[sceneDelegate] sceneDidBecomeActive
<scene> didActivate Notification
<app> didBecomeActive Notification

Show Dock and do "Show All Windows" from dock icon,

[sceneDelegate] sceneWillResignActive
<scene> willDeactivate Notification
<app> willResignActive Notification

Select another scene,

[sceneDelegate] sceneWillEnterForeground
<scene> sceneEnterForeground Notification
[sceneDelegate] sceneDidBecomeActive
<scene> didActivate Notification
<app> didBecomeActive Notification
[sceneDelegate] sceneDidEnterBackground
<scene> didEnterBackground Notification

Summarize the above:

• Methods in AppDelegate, e.g. applicationDidBecomeActive, was never called.
• Notifications in UIApplication, e.g. UIApplication.didBecomeActiveNotification was sent!! 😲
• Notifications in UIScene, e.g. UIScene.didActivateNotification was sent.
• Note that when switching one scene to another scene in same app, UIApplication.willEnterForegroundNotification and UIApplication.didEnterBackgroundNotification were not sent contrary to that corresponding scene event was sent. ⚠️