The focus-existing (and navigate-existing) do not actually focus the window

[mgiuca]

In the algorithm to prepare a web app launch client, we should explicitly say to focus the window. (This is not an implementation detail left to the user agent; it's something that other specs, notably window.focus(), can do.)

We should cargo-cult this off the window.focus() algorithm which looks like it primarily invokes an algorithm called focusing steps but there may be details from window.focus() that we want to use as well. (I'd like to briefly understand focusing steps before blindly calling it.)

[alancutter]

For now I've cargo culted off the notification API that more handwavily just says to bring it in focus: https://notifications.spec.whatwg.org/#activating-a-notification