Proposal: App registration upon availability of other Apps

[fahad19]

Currently

If we want to wait for certain Apps to be available first, before registering a new App, we do it like this:

window.app.getAppOnceAvailable$('FooApp').subscribe(
  fooApp => window.app.registerApp(BazApp)
);

Proposal

We can make things a bit more easier:

window.app.registerAppOnceAvailable(['FooApp', 'BarApp'], BazApp);

(not convinced with the method name registerOnceAvailable yet)

Relates to #425