AppView
AppView 是一个基本实现，提供了绘制有主见的 application view 的功能。继承 BoxView。
AppView is a base implementation providing functionality to draw opinionated application view. Inherits BoxView.
通常 MenuBarView 和 StatusBarView 分别是菜单和状态视图的一般想法。主内容视图则是用户想要显示在其中的任何内容。
Generic idea is to have menu and status views which typically are MenuBarView and StatusBarView respectively. Main content view is then whatever user want to show in it.
┌──────────────────────────┐
│ Menu │
├──────────────────────────┤
│ │
│ Main │
│ │
├──────────────────────────┤
│ Status │
└──────────────────────────┘Key Handling
如果菜单有焦点，则在那里处理键处理，然后咨询主要内容来进行处理。最后，处理光标左右以调度 AppViewEvent。
If menu has a focus key handling is processed there, then main is consulted for handling. Lastly cursor left/right are processed to dispatch AppViewEvent.