* chore: add WebContentsView.webContents helper
* chore: no need to handle webContents option
* chore: Create WebContentsView in C++
* chore: make WebContentsView accept web_preferences
* fix: nativeWindowOpen still passes WebContents to BrowserWindow
* chore: no more need of WebContentsViewRelay
* test: WebContentsView now takes options
* fix: avoid creating 2 constructors
* docs: add references to app.whenReady() in isReady
* refactor: prefer app.whenReady()
In the docs, specs, and lib, replace instances of `app.once('ready')`
(seen occasionally) and `app.on('ready')` (extremely common) with
`app.whenReady()`.
It's better to encourage users to use whenReady():
1. it handles the edge case of registering for 'ready' after it's fired
2. it avoids the minor wart of leaving an active listener alive for
an event that wll never fire again
