View naming in Navigator API could be more 'statically typed'

Originally by @emarc

In the new Navigator API, Views are registered using a string, and the API does not help in handling these. This is not ideal.

However, it's not clear how the problem could be adressed; the fragment will obviously always be a String.

One convenient practice is to add a static to each View: MyView.NAME, but this can't be APIfied.

Adding View.getViewName() would remove the need to tell the Navigator explicitly when adding instances, i.e navigator.addView(new MyView()), but would not work when adding classes as navigator.addView("name", MyView.class). One possibility is to provide all of addView(View), addView(String, View), addView(String, Class), where the latter two would 'override' getViewName() - this might be overly complex, though.

Imported from https://dev.vaadin.com/ issue #9057

Start by examining the Navigator API and its existing addView overloads, especially the string- and class-based registration described in the issue. Determine and agree on a naming approach for view instances and classes; done means the API design is decided and consistently specified.