docs: views API: child views and layout engine

[zcbenz]

This specs defines APIs to add/remove child views, and proposes to use YogaLayout as layout engine.

This is an example that implements a text editor with sidebar:

(The Button and TextEdit classes are not yet defined and only for demonstration purpose.)

const win = new TopLevelWindow({})

// The content view has its children arranged horizontally.
const content = new View()
content.setStyle({flexDirection: 'row'})
win.setContentView(content)

// The sidebar is a child of content view and has 5px paddings.
const sidebar = new View()
sidebar.setStyle({padding: 5})
contentView.addChildView(sidebar)

// The buttons in the sidebar, they shows images instead of text.
const open = new Button('')
open.setImage('open@2x.png')
open.setStyle({marginBottom: 5})
sidebar.addChildView(open)
const save = new Button('')
save.setImage('open@2x.png')
sidebar.addChildView(save)

// Make the sidebar have a fixed width which is enough to show all the buttons.
sidebar.setStyle({width: sidebar.getPreferredSize().width})

// The text edit view would take all remaining spaces.
const edit = new TextEdit()
edit.setStyle({flex: 1})
contentView.addChildView(edit)

[zcbenz]

Closing for now, will prepare a new spec.