Re: [w3c/manifest] Add installation prompt control flow (#417)

Ok, so  #506 revealed a couple of issues that we could address. This aim is to minimize impact on existing developer code... but the following would be a breaking change.  
 
I'm wondering, can we make the design work more like this:

```JS
window.onbeforeinstallprompt = async function ( e )  {
  if (!userIsDoingThings) {
    return; // automatically show it.  
  }
  e.preventDefault(); 
  await Promise.all(thingsTheUserIsDoingThatBlockInstall);
  const promptResult = await ev.prompt();
  switch (promptResult.userChoice) {
    case "dismissed":
      // The user didn't want the app 😢
      analytics({ userHatesUs: true });
      break;
    case "accepted":
      // Awesome! it's installing 
      analytics({ userHatesUs: false });
      break;
    default:
      console.error(`Unknown choice? ${promptResult.userChoice}`);
  }
}

window.oninstall = e => {
  // The application installed!
}
```

Key things that could change: 

 1. get rid of .userChoice attribute on the event. 
 1. define a new `interface PromptResult` that contains the user choice and maybe other stuff. 


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