Proposal: Check availability of multiple Apps

## Currently

We can trigger something as soon as our desired App is available, via this API:

```js
rootApp.getAppOnceAvailable$('SomeAppName')
.subscribe(someApp => console.log(someApp));
```

## Proposal

It would be handy if we can check the availability of multiple apps together:

```js
rootApp.getAppOnceAvailable$(['SomeApp', 'AnotherApp'])
.subscribe(function (someApp, anotherApp) {
// fires once, when both `SomeApp` and `AnotherApp` are available
});
```

The proposal names rootApp.getAppOnceAvailable$ as the entry point; start by reading the existing single-app behavior and its availability semantics. Determine how multiple app results should be delivered when all requested apps are available, and consider the API's completion criteria and test coverage; no files or tests are named in the issue.

Written by the indexing model from the issue text.